How does Facebook determine which Thumbnail to display when you attach a link to a wall post?

Patrick: How does Facebook determine which Thumbnail to display when you attach a link to a wall post?
When sharing a link through a wall post, Facebook grabs images from the destination site. How are these images determined and can you set parameters on your site to allow Facebook to get certain images or block certain images?

Answers and Views:

Answer by alextheboy1
it automatically takes any of the pictures they can find from the website (unless ther is no pix on the site in which case u cannot show a pic)