I am also having thumbnail related issues when working with one of my sites.
When I put a link to a post into Facebook, it shows the Gravatar logo.
So if I link to this post: http://www.casualvoyeur.com/hot-girls-kissing-videos -- FB shows the Gravatar logo.
When I put the home URL into Facebook, it shows video thumbnails from a specific post...
This is weird to say the least.
Another Note: When I used the wordbooker plugin, it was able to pull thumbnails from the videos in the post automatically. I cannot continue to use that plugin, however, because it includes post excerpts which I do not wish to include in a facebook post.