diff --git a/content/awards/awards.md b/content/awards/awards.md
index b1f0f5e9cfc..20d00d44b85 100644
--- a/content/awards/awards.md
+++ b/content/awards/awards.md
@@ -105,7 +105,7 @@ Thanks to our amazing community, FORRT has become an ***award-winning*** grassro
 
 ### [FORWARD: A UNESCO Decade of Science Programme](../educators-corner/023-forward-a-unesco-decade-of-science-programme/)
 
-<img src="unesco_recognition.webp" width="200" align="left" style="margin-right: 10px;" alt="FORWARD: A UNESCO Decade of Science Programme"> 
+<img src="unesco_recognition.webp" width="200" height="207" align="left" style="margin-right: 10px;" alt="FORWARD: A UNESCO Decade of Science Programme"> 
 
 **FORRT** is one of the few initiatives worldwide to be formally endorsed as a **Programme** of the [International Decade of Sciences for Sustainable Development (IDSSD, 2024–2033)](https://www.un-sciences-decade.org/en/endorsed-activities/forward-forrt-world-development-fostering-open-research-reproducibility-worldwide-accessible?hub=63) , led by UNESCO and the United Nations (UN). This endorsement reflects the highest level of UN policy recognition and positions FORRT’s work in Education & Pedagogy (Curriculum Hub), Metascience & Research (Replication Hub) and Social Justice & DEIA as an **essential contribution to the UN’s strategy for achieving the Sustainable Development Goals (SDGs)**.
 
diff --git a/content/home/5-achievements.md b/content/home/5-achievements.md
index 362c033db55..1bf7ec9e1aa 100644
--- a/content/home/5-achievements.md
+++ b/content/home/5-achievements.md
@@ -23,7 +23,7 @@ title = "Achievements &amp; Awards"
 
 ### [FORWARD: A UNESCO Decade of Science Programme](../educators-corner/023-forward-a-unesco-decade-of-science-programme/)
 
-<img src="../awards/unesco_recognition.webp" width="200" align="left" style="margin-right: 10px;" alt="FORWARD: A UNESCO Decade of Science Programme"> 
+<img src="../awards/unesco_recognition.webp" width="200" height="207" align="left" style="margin-right: 10px;" alt="FORWARD: A UNESCO Decade of Science Programme"> 
 
 **FORRT** is one of the few initiatives worldwide to be formally endorsed as a **Programme** of the [International Decade of Sciences for Sustainable Development (IDSSD, 2024–2033)](https://www.un-sciences-decade.org/en/endorsed-activities/forward-forrt-world-development-fostering-open-research-reproducibility-worldwide-accessible?hub=63) , led by UNESCO and the United Nations (UN). This endorsement reflects the highest level of UN policy recognition and positions FORRT’s work in Education & Pedagogy (Curriculum Hub), Metascience & Research (Replication Hub) and Social Justice & DEIA as an **essential contribution to the UN’s strategy for achieving the Sustainable Development Goals (SDGs)**.
 
diff --git a/content/home/5-get-involved.md b/content/home/5-get-involved.md
index fc603108271..d469cb4790f 100644
--- a/content/home/5-get-involved.md
+++ b/content/home/5-get-involved.md
@@ -23,7 +23,7 @@ title = "How can you help?"
 #  icon = "users"
 #  icon_pack = "fas"
   img = "teamwork.webp"
-  imgalttext = "Image of a megaphone"
+  imgalttext = "Several hands stacked on top of each other. Illustration."
   name = "Join Our Teams"
   description = "Everything we develop and produce is in the open and open to anyone willing to help. Check out our [several teams here](/about/teams) and our [get-involved page](/about/get-involved) too."
 
diff --git a/layouts/_default/_markup/render-image.html b/layouts/_default/_markup/render-image.html
new file mode 100644
index 00000000000..e86fb9e2f2b
--- /dev/null
+++ b/layouts/_default/_markup/render-image.html
@@ -0,0 +1,28 @@
+{{/* Markdown image hook: reproduce Hugo's default <img> and add intrinsic width/height
+     so the browser reserves the image's box before it loads. The site stylesheet sets
+     `img { height: auto; max-width: 100% }`, so the attributes fix the aspect ratio
+     without freezing the rendered size or breaking responsive scaling.
+
+     Dimensions come from the page bundle's own resource first, then from static/, and
+     are skipped for remote images and for formats Hugo cannot measure (SVG). */}}
+{{- $dest := .Destination -}}
+{{- $dims := "" -}}
+{{- $raster := slice "png" "jpeg" "gif" "webp" -}}
+{{- if not (or (strings.HasPrefix $dest "http") (strings.HasPrefix $dest "//") (strings.HasPrefix $dest "data:")) -}}
+  {{- with .PageInner.Resources.Get (strings.TrimPrefix "./" $dest) -}}
+    {{/* Match Hugo's default hook, which links page-bundle images by permalink rather
+         than by the author's page-relative path. */}}
+    {{- $dest = .RelPermalink -}}
+    {{- if in $raster .MediaType.SubType -}}
+      {{- $dims = printf "width=\"%d\" height=\"%d\"" .Width .Height -}}
+    {{- end -}}
+  {{- else -}}
+    {{- $static := printf "static/%s" (strings.TrimPrefix "/" $dest) -}}
+    {{- if and (fileExists $static) (in (slice ".png" ".jpg" ".jpeg" ".gif" ".webp") (path.Ext $static | lower)) -}}
+      {{- with imageConfig $static -}}
+        {{- $dims = printf "width=\"%d\" height=\"%d\"" .Width .Height -}}
+      {{- end -}}
+    {{- end -}}
+  {{- end -}}
+{{- end -}}
+<img src="{{ $dest | safeURL }}" alt="{{ .PlainText }}"{{ with .Title }} title="{{ . }}"{{ end }}{{ with $dims }} {{ . | safeHTMLAttr }}{{ end }}>
diff --git a/layouts/partials/chatbot.html b/layouts/partials/chatbot.html
index 8359b0d0fb9..543f0de65b0 100644
--- a/layouts/partials/chatbot.html
+++ b/layouts/partials/chatbot.html
@@ -901,7 +901,7 @@
   {{ if $active }}
   <!-- Toggle Button -->
   <button id="just-os-btn" onclick="toggleJustOS()" aria-label="Open Chatbot">
-    <img id="just-os-img" src="/img/just_os_icon.webp" alt="Chat with us">
+    <img id="just-os-img" src="/img/just_os_icon.webp" {{ partial "functions/static_img_dims" "just_os_icon.webp" }} alt="Chat with us">
     <div id="just-os-bubble">
       How can I help you?
     </div>
@@ -933,7 +933,7 @@
   {{ else }}
   <!-- Inactive state: icon links to the standalone page where the full offline notice lives -->
   <a id="just-os-btn" href="/just_os_chatbot/" aria-label="JUST-OS chatbot is offline — open status page">
-    <img id="just-os-img" src="/img/just_os_icon.webp" alt="JUST-OS chatbot (offline)">
+    <img id="just-os-img" src="/img/just_os_icon.webp" {{ partial "functions/static_img_dims" "just_os_icon.webp" }} alt="JUST-OS chatbot (offline)">
     <span id="just-os-offline-badge" aria-hidden="true" title="Chatbot offline">
       <!-- Unplugged plug icon -->
       <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.2" stroke-linecap="round" stroke-linejoin="round">
diff --git a/layouts/partials/functions/logo_dims.html b/layouts/partials/functions/logo_dims.html
new file mode 100644
index 00000000000..19562120484
--- /dev/null
+++ b/layouts/partials/functions/logo_dims.html
@@ -0,0 +1,23 @@
+{{/* Intrinsic width/height attributes for the site logo, so the navbar reserves the
+     brand's box before the image loads. The stylesheet sizes the logo by height with
+     `width: auto`, which leaves its width unknown until the file arrives and shifts
+     the menu sideways on first paint.
+     Input: the logo resource (as returned by functions/get_logo).
+     Output: `width="W" height="H"` (safe attribute string), or empty when unavailable.
+
+     Hugo exposes .Width/.Height on raster resources only, so the SVG's aspect ratio is
+     read from the first viewBox in the file, which is the root <svg> element's. */}}
+{{ $dims := "" }}
+{{ with . }}
+  {{ if eq (path.Ext .Name | lower) ".svg" }}
+    {{ with (findRE `viewBox="[^"]+"` .Content 1) }}
+      {{ $nums := findRE `[0-9]+(\.[0-9]+)?` (index . 0) }}
+      {{ if eq (len $nums) 4 }}
+        {{ $dims = printf "width=\"%.0f\" height=\"%.0f\"" (float (index $nums 2)) (float (index $nums 3)) }}
+      {{ end }}
+    {{ end }}
+  {{ else }}
+    {{ $dims = printf "width=\"%d\" height=\"%d\"" .Width .Height }}
+  {{ end }}
+{{ end }}
+{{ return ($dims | safeHTMLAttr) }}
diff --git a/layouts/partials/navbar.html b/layouts/partials/navbar.html
index d08b8043ea8..eb6011c4b93 100644
--- a/layouts/partials/navbar.html
+++ b/layouts/partials/navbar.html
@@ -4,8 +4,10 @@
 {{ $show_logo := site.Params.main_menu.show_logo | default true }}
 {{ $has_logo := fileExists "assets/images/logo.webp" | or (fileExists "assets/images/logo.svg") }}
 {{ $logo := "" }}
+{{ $logo_dims := "" }}
 {{ if $has_logo }}
 {{ $logo = (partial "functions/get_logo" (dict "constraint" "max_height" "size" 70)) }}
+{{ $logo_dims = partial "functions/logo_dims" $logo }}
 {{ end }}
 
 <nav class="navbar navbar-expand-lg navbar-light compensate-for-scrollbar" id="navbar-main">
@@ -15,7 +17,7 @@
     <div class="d-none d-lg-inline-flex">
       <a class="navbar-brand" href="{{ "/" | relLangURL }}">
         {{- if $has_logo -}}
-        <img src="{{ $logo.RelPermalink }}" alt="{{ site.Title }} Logo. Link to homepage">
+        <img src="{{ $logo.RelPermalink }}" {{ $logo_dims }} alt="{{ site.Title }} Logo. Link to homepage">
         {{- else -}}
         {{- site.Title -}}
         {{- end -}}
@@ -34,7 +36,7 @@
     <div class="navbar-brand-mobile-wrapper d-inline-flex d-lg-none">
       <a class="navbar-brand" href="{{ "/" | relLangURL }}">
         {{- if $has_logo -}}
-        <img src="{{ $logo.RelPermalink }}" alt="{{ site.Title }}">
+        <img src="{{ $logo.RelPermalink }}" {{ $logo_dims }} alt="{{ site.Title }}">
         {{- else -}}
         {{- site.Title -}}
         {{- end -}}
diff --git a/layouts/shortcodes/forward_unesco_sdgs_card.html b/layouts/shortcodes/forward_unesco_sdgs_card.html
index 9b7e07bd493..09a3c66e89a 100644
--- a/layouts/shortcodes/forward_unesco_sdgs_card.html
+++ b/layouts/shortcodes/forward_unesco_sdgs_card.html
@@ -7,22 +7,22 @@ <h3>FORWARD (FORRT for World Development)</h3>
 
         <div class="sdg-icons-wrapper">
             <a href="https://sdgs.un.org/goals/goal4" target="_blank" rel="noopener noreferrer">
-                <img src="/img/forward_unesco_sdgs/4.webp" alt="SDG 4: Quality Education"
+                <img src="/img/forward_unesco_sdgs/4.webp" {{ partial "functions/static_img_dims" "forward_unesco_sdgs/4.webp" }} alt="SDG 4: Quality Education"
                     title="SDG 4: Quality Education">
             </a>
             <a href="https://sdgs.un.org/goals/goal5" target="_blank" rel="noopener noreferrer">
-                <img src="/img/forward_unesco_sdgs/5.webp" alt="SDG 5: Gender Equality" title="SDG 5: Gender Equality">
+                <img src="/img/forward_unesco_sdgs/5.webp" {{ partial "functions/static_img_dims" "forward_unesco_sdgs/5.webp" }} alt="SDG 5: Gender Equality" title="SDG 5: Gender Equality">
             </a>
             <a href="https://sdgs.un.org/goals/goal10" target="_blank" rel="noopener noreferrer">
-                <img src="/img/forward_unesco_sdgs/10.webp" alt="SDG 10: Reduced Inequalities"
+                <img src="/img/forward_unesco_sdgs/10.webp" {{ partial "functions/static_img_dims" "forward_unesco_sdgs/10.webp" }} alt="SDG 10: Reduced Inequalities"
                     title="SDG 10: Reduced Inequalities">
             </a>
             <a href="https://sdgs.un.org/goals/goal16" target="_blank" rel="noopener noreferrer">
-                <img src="/img/forward_unesco_sdgs/16.webp" alt="SDG 16: Peace, Justice and Strong Institutions"
+                <img src="/img/forward_unesco_sdgs/16.webp" {{ partial "functions/static_img_dims" "forward_unesco_sdgs/16.webp" }} alt="SDG 16: Peace, Justice and Strong Institutions"
                     title="SDG 16: Peace, Justice and Strong Institutions">
             </a>
             <a href="https://sdgs.un.org/goals/goal17" target="_blank" rel="noopener noreferrer">
-                <img src="/img/forward_unesco_sdgs/17.webp" alt="SDG 17: Partnerships for the Goals"
+                <img src="/img/forward_unesco_sdgs/17.webp" {{ partial "functions/static_img_dims" "forward_unesco_sdgs/17.webp" }} alt="SDG 17: Partnerships for the Goals"
                     title="SDG 17: Partnerships for the Goals">
             </a>
         </div>
diff --git a/scripts/check_seo_html.py b/scripts/check_seo_html.py
index 1b0e5b7ea98..2d4271e1fa4 100644
--- a/scripts/check_seo_html.py
+++ b/scripts/check_seo_html.py
@@ -11,9 +11,11 @@
      not count; language-mermaid blocks belong to Mermaid), and mermaid.min.js
      exactly on pages with a language-mermaid block.
   4. Every <img> carries an alt attribute (a bare `alt` counts as present).
+  5. Every homepage <img> carries both width and height, so the browser reserves
+     its box before the file loads (#869).
 
 Report-only sections (never fail): pages with zero or several <h1>, and the
-count of homepage images that carry both width and height.
+site-wide share of images that carry both width and height.
 """
 
 import os
@@ -87,6 +89,11 @@ def has_mermaid_code(self):
     def imgs_without_alt(self):
         return [img for img in self.imgs if "alt" not in img]
 
+    @property
+    def imgs_without_dims(self):
+        return [img for img in self.imgs
+                if "width" not in img or "height" not in img]
+
 
 def load_pages(public_dir):
     pages = []
@@ -127,6 +134,12 @@ def check_conditional_assets(pages):
     }
 
 
+def check_homepage_img_dims(home):
+    if home is None:
+        return ["index.html (homepage not found)"]
+    return [img.get("src", "(no src)") for img in home.imgs_without_dims]
+
+
 def check_img_alt(pages):
     return ["%s (%d image(s))" % (p.path, len(p.imgs_without_alt))
             for p in pages if p.imgs_without_alt]
@@ -149,6 +162,7 @@ def main(public_dir):
         '1. no <h1>Search</h1>': check_no_search_heading(pages),
         "2. homepage free of mermaid/highlight.js assets": check_homepage_assets(home),
         "4. every <img> has an alt attribute": check_img_alt(pages),
+        "5. every homepage <img> has width and height": check_homepage_img_dims(home),
     }
     conditional = check_conditional_assets(pages)
     results["3. conditional highlight.js / mermaid loading"] = [
@@ -167,14 +181,17 @@ def main(public_dir):
                 if items:
                     print_items(reason, items)
         else:
-            print_items("pages", results[title])
+            # Check 5 lists offending image sources, every other check lists pages.
+            print_items("images" if title.startswith("5.") else "pages", results[title])
 
     print("\nReport only (never fails)")
     print_items("pages with no <h1>", [p.path for p in pages if not p.h1_texts])
     print_items("pages with several <h1>", [p.path for p in pages if len(p.h1_texts) > 1])
-    if home is not None:
-        sized = [i for i in home.imgs if "width" in i and "height" in i]
-        print("  homepage images: %d, with width and height: %d" % (len(home.imgs), len(sized)))
+    all_imgs = sum(len(p.imgs) for p in pages)
+    unsized = sum(len(p.imgs_without_dims) for p in pages)
+    if all_imgs:
+        print("  images site-wide: %d, with width and height: %d (%.1f%%)"
+              % (all_imgs, all_imgs - unsized, 100.0 * (all_imgs - unsized) / all_imgs))
 
     return 1 if any(results.values()) else 0