How to extract images from a webpage as Facebook does?

[onix]

If I insert in my wall a link like this:http://blog.bonsai.tv/news/il-nuovo-vezzo-della-lega-nord-favorire-i-lombardi-alluniversita/then facebook extract the image in the post and not the first image in the webpage ( not image logo or other little images for example ) !!How facebook does that ?