Facebook share in drupal7 sharethis module picking wrong thumbnail image
Did you test the page with the Facebook Debugger?  Remember that Facebook caches page info, so changes don't appear immediately.  A side effect of the debugger is that it clears the cache when you test a URL, so changes get picked up.

If you wan to use a specific image, you need to be sure it explicitly has an og:image element in the header.  I am pretty sure this is something that the Meta Tags module can do this for you.