Yes, you can make a thumbnail unrelated to the picture. It will send your hitcount through the roof. People do it all the time, usually to make some kind of point. Someone posted a fractal with a sexy girl thumbnail. The image had a humongous hitcount, and he seemed to think it was proof that "only the tits get the hits." Personally, I think at least half the hits were because people thought they had clicked on the wrong link, and kept trying again. I know I did. ;-)