Hey,
The first "significant" image on the page, ie one that can be used to "represent" the page. I realize you can not do this fully accurate without having some additional s*mantics, but if it works most of the time, such as is the case with my approach, it's good enough.
Cheers
-- Jeroen De Dauw http://www.bn2vs.com Don't panic. Don't be evil. --