Add default social preview (og:image) to the site

_config.yml

@@ -15,6 +15,7 @@ baseurl: "" # the subpath of your site, e.g. "/blog"
 gh_repo: github/training-kit
 repository: github/training-kit
 teaser: # filename of teaser fallback teaser image placed in /images/, .e.g. "500x300.png"
+og_image: og-image.png # default social preview image placed in /images/
 breadcrumbs: true # true, false (default)
 words_per_minute: 200
 

_includes/seo.html

@@ -55,26 +55,26 @@
 
 {% if site.twitter.username %}
   <meta name="twitter:site" content="@{{ site.twitter.username | replace: "@", "" }}">
-  <meta name="twitter:title" content="{{ page.title | default: site.title | markdownify | strip_html | strip_newlines | escape_once }}">
-  <meta name="twitter:description" content="{{ seo_description }}">
-  <meta name="twitter:url" content="{{ canonical_url }}">
+{% endif %}
 
-  {% if page.header.image %}
-    <meta name="twitter:card" content="summary_large_image">
-    <meta name="twitter:image" content="{% if page.header.image contains "://" %}{{ page.header.image }}{% else %}{{ page.header.image | prepend: "/images/" | prepend: base_path }}{% endif %}">
-  {% else %}
-    <meta name="twitter:card" content="summary">
-    {% if site.og_image %}
-      <meta name="twitter:image" content="{{ site.og_image | prepend: "/images/" | prepend: base_path }}">
-      <meta property="og:image" content="{{ site.og_image | prepend: "/images/" | prepend: base_path }}">
-    {% endif %}
-  {% endif %}
+<meta name="twitter:title" content="{{ page.title | default: site.title | markdownify | strip_html | strip_newlines | escape_once }}">
+<meta name="twitter:description" content="{{ seo_description }}">
+<meta name="twitter:url" content="{{ canonical_url }}">
 
-  {% if seo_author_twitter %}
-    <meta name="twitter:creator" content="@{{ seo_author_twitter }}">
+{% if page.header.image %}
+  <meta name="twitter:card" content="summary_large_image">
+  <meta name="twitter:image" content="{% if page.header.image contains "://" %}{{ page.header.image }}{% else %}{{ page.header.image | prepend: "/images/" | prepend: base_path }}{% endif %}">
+{% else %}
+  <meta name="twitter:card" content="summary">
+  {% if site.og_image %}
+    <meta name="twitter:image" content="{{ site.og_image | prepend: "/images/" | prepend: base_path }}">
   {% endif %}
 {% endif %}
 
+{% if seo_author_twitter %}
+  <meta name="twitter:creator" content="@{{ seo_author_twitter }}">
+{% endif %}
+
 {% if site.facebook %}
   {% if site.facebook.publisher %}
     <meta property="article:publisher" content="{{ site.facebook.publisher }}">
@@ -87,6 +87,8 @@
 
 {% if page.header.image %}
   <meta property="og:image" content="{% if page.header.image contains "://" %}{{ page.header.image }}{% else %}{{ page.header.image | prepend: "/images/" | prepend: base_path }}{% endif %}">
+{% elsif site.og_image %}
+  <meta property="og:image" content="{{ site.og_image | prepend: "/images/" | prepend: base_path }}">
 {% endif %}
 
 {% if page.date %}