Possible Duplicate:
How does Facebook Sharer select Images?
When I attach a link on facebook I get the title, url, description and images from that page.
But facebook separates the images and gives just the important ones. And this is what I don't understand.
For example, attach this link
http://tonlinegames.com/ - This is a gaming site with photos about the games.
When you attach it facebook will give you as results only the image about the games, but there are a lots of other images like buttons and so one.
Facebook makes use of Open Graph meta tags when available:
http://developers.facebook.com/docs/opengraph
Otherwise, it probably just uses common sense heuristics (title, h1, p tags, large images, etc).