Hoople - 8:12 pm on Jul 21, 2011 (gmt 0)

Facebook pulls the info from the page meta property og tage content. Examplified below:

<meta property="og:title" content="title text" />
<meta property="og:type" content="non_profit" />
<meta property="og:url" content="http://www.example.com/" />
<meta property="og:image" content="http://www.example.com/images/image.jpg" />
<meta property="og:site_name" content="Site name" />
<meta property="fb:admins" content="#*$!000#*$!000" />

This is documented in the section on placing a share/like button on a webpage in the FaceBook developers help section.