I think that the thumbnail is the marketing piece. It depends on the image, but I try to make it as intersting as possible. Sometimes it's the whole image, other times it's part of it. I usually start with a 200 x 200 square, and look for a good crop, if that fails, then I crop it by eye, and if that still fails, you get the while thing. Like Kate, I get annoyed when I click on a thumb that is a minute detail. Usually it's because the thumb looks so cool, I need to see it, and I find out that the cool part is just a little part of the corner and the rest of the image sucks. I want the hits, but I don't want to deceive the visitor. If were intersted in deception, I'd make a thumbnail from images obtained from one of the many porn sites, blur out the naughty bits and check off the nudity box. That's a guarantee you'll get hits.