I saw this in a page and was trying to figure out what this was used for..
[link rel="image_src" href="linktoimage.jpg" /]
Basically, when you submit/share this to Facebook , this helps Facebook determine which image to put next to the link. If this is not present, Facebook tries to put in the first image it finds on the page, which may not be the best one to represent your site. (NOTE: Facebook also uses your meta title and meta description tags.)
I am hoping that someday that search engines will use this to put thumbnails of this image in their results display... But oh well. Until then, it will help with social sharing. Since Facebook is such a large resource, this is currently an important thing to consider.
Optimizing your site for Facebook
Enhanced Social Share
Facebook Meta Tags