Erwin Olaf and Hans van Manen (who is also a choreographer) are probably the best and most famous gay photographers living in Amsterdam these days. Both were influenced by Robert Mapplethorpe. Their books might be sold out, but you can always find picture cards with there work in gay the bookstore Vrolijk (Paleisstraat 135, tel. 623.51.42).
The Melkweg gallery (Lijnbaansgracht 234/A, tel. 624.17.77) often has gay and lesbian photo exhibitions.
Lesbian photographers are Gon Buurman and Diana Blok. The latter had a quite interesting exhibition of photographs of transvestites, a few years ago.
For those of you interested in photograpy in general: Amsterdam has a great photograpy museum:
Foam, Keizersgracht 609