I've just re-skinned my old blog using a child theme based on Twenty Twelve. http://yphotography.co.uk
I've set the default post type as Image, to put the headings at the bottom, I have also messed about a little bit with the default layout, as you do.
When you post a link to a blog post in Facebook, it picks the header PNG as a thumbnail. If you re-post, it gets it right the second time.
Is there any way to convince Facebook to ignore the header and grab the top photo first time?