lucy24 - 11:41 pm on Feb 23, 2012 (gmt 0)
I'm not privvy to the details of the thumbnail generation they use, so I can't answer that question.
If you are on FB and have a www page with images, you can easily test it. Look at your stats after setup and you will see a "facebookexternalhit" UA picking up all images. Now get a few friends (either real ones or FB "friends") to visit the page containing the thumbnail you picked in the previous step. Look at your stats again and you'll see the hotlinks.