This is the template code that pertains to Facebook open graph meta data and images:
<xen:if is="{$avatar}"><meta property="og:image" content="{$avatar}" /></xen:if>
<meta property="og:image" content="{xen:helper fullurl, @ogLogoPath, 1}" />
Default behaviour means you have two choices, either the avatar of the person sharing or the image as specified by Style Properties.
See the problem is, Facebook doesn't actually look at the content of your page, it has to be told what's on it.
Common misconception that Facebook actually looks at the content and makes the post based on that.
Look at the code for this very page:
Code:
<meta property="og:site_name" content="XenForo Community" />
<meta property="og:image" content="http://xenforo.com/community/data/avatars/m/3/3030.jpg?1294984494" />
<meta property="og:image" content="http://xenforo.com/community/styles/default/xenforo/logo.og.png" />
<meta property="og:type" content="article" />
<meta property="og:url" content="http://xenforo.com/community/threads/why-facebook-share-doesnt-see-images-included-in-the-post.36593/" />
<meta property="og:title" content="why facebook share doesn't see images included in the post" />
<meta property="fb:app_id" content="146201428725181" />
This is information specifically fed to Facebook via the meta tags.
There is one solution that might help... let me track it down.
EDIT:
Yep, check out this post from Jake Bunce:
http://xenforo.com/community/thread...humbnail-for-facebook-feed.18284/#post-237150
Or this post from Mutt:
http://xenforo.com/community/thread...humbnail-for-facebook-feed.18284/#post-321987
These both override the default behaviour in different ways.
Mutt's may be closer to what you're after.