fix: resume the preview image SEO tag (#529)

Fix #529

_includes/head.html

@@ -23,7 +23,30 @@
 
   {% endif %}
 
-  {% seo title=false %}
+  {% capture seo_tags %}
+    {% seo title=false %}
+  {% endcapture %}
+
+  {% if site.img_cdn and seo_tags contains 'og:image' %}
+    {% assign properties = 'og:image,twitter:image' | split: ',' %}
+
+    {% for prop in properties %}
+      {% if site.img_cdn contains '//' %}
+      <!-- `site.img_cdn` cross-origin URL -->
+        {% capture target %}<meta property="{{ prop }}" content="{{ site.url }}{% endcapture %}
+        {% capture replacement %}<meta property="{{ prop }}" content="{{ site.img_cdn }}{% endcapture %}
+      {% else %}
+        <!-- `site.img_cdn` is a local file path -->
+        {% capture target %}<meta property="{{ prop }}" content="{{ site.url }}{{ site.baseurl }}{% endcapture %}
+        {% assign replacement = target | append: site.img_cdn %}
+      {% endif %}
+
+      {% assign seo_tags = seo_tags | replace: target, replacement %}
+
+    {% endfor %}
+  {% endif %}
+
+  {{ seo_tags }}
 
   <title>
   {%- unless page.layout == "home" -%}

_layouts/post.html

@@ -11,11 +11,11 @@ tail_includes:
 
 {% include lang.html %}
 
-{% if page.image.src %}
+{% if page.image.path %}
   {% capture bg %}
     {% unless page.image.no_bg %}{{ 'bg' }}{% endunless %}
   {% endcapture %}
-  <img src="{{ page.image.src }}" class="preview-img {{ bg | strip }}"
+  <img src="{{ page.image.path }}" class="preview-img {{ bg | strip }}"
       alt="{{ page.image.alt | default: "Preview Image" }}"
 
       {% if page.image.width %}