Peter's right, Alan. And the other thing to note is that, relative to painting, photography is a very young art. History's great paintings have been culled by time and public interest to a very few. Consider that at the time the Impressionists were being spurned by the art powers in France, the painters whose work those same powers were pushing are now pretty much known only to art historians. The culling process is still going on for photographs. I think we're just about there for Atget's work, but we haven't finished the job for people like Cartier-Bresson or Ansel Adams, and certainly not for their successors.
(We're also haven't yet culled Jackson Pollock's drippings.)