I think it's all about historical context, who's famous, and the story behind the art as well as the aesthetics of the picture itself. Duchamp's urinal is worth more than the one I peed in this morning because there's a great story and some historical context/significance to go along with it. We just can't get enough of the Stieglitz/O'Keefe story or the Weston/Wilson thing either. Weston, Stieglitz, Steichen... all fabulous characters as well as great artists. People pay for that kind of thing.