Besides the photo collection at SF MOMA on 3d St, don't miss the major galleries specializing in photography at 49 Geary nr Market (Fraenkel, Koch, Scot Nichol, & often Wirtz). There's also a sort of co-op photo gallery on 2d flr.
(RayKo is interesting, but it's a photo center that shows amateur/'undiscovered' work.)
Besides the major SFMOMA photo shows, there's always a selection from the collection on exhibit.
Kirk