For best practices for uploading gravatars, avatars and blavatars scroll down to below the video here: http://onecoolsitebloggingtips.com/2010/01/24/gravatar-avatar-blavatar/

Note: Gravatar images are cached and frequently take several days to display after a change. Do not keep uploading! The first place they tend to display is here in the forums. If several days have passed and that change is not in evidence then post here for Gravatar support > http://en.gravatar.com/site/contact-us/

Unfortunately, we have no control over the images that Facebook chooses to display or when they choose to flush their image caches. We really only send them the URL of the post, and they hunt through that for the excerpt and images to include. From: http://en.forums.wordpress.com/topic/avalaible-facebook-thumbnail-issue?replies=7#post-1000734
Read also: http://developers.facebook.com/tools/debug
from: http://en.forums.wordpress.com/topic/fb-link-has-no-thumbnails?replies=4#post-936861