Photographers Direct (a UK based stock photography company)
I have been using Photographers Direct for my own research on the distribution of a certain plant that is sometimes visible in photographs taken in scenic wet locations (especially waterfalls). For me, it is worthwhile to pay the photographer concerned for use rights, because there is little chance I will reach the location myself.
The company running this service has a good system for photographers based in less wealthy regions of the world.
In its own words, the company has "pioneered the concept of Fair Trade Stock Photography" This makes the range of photography more interesting than at many commercially-orientated sites. The company currently has 2,302,523stockimages in its search engine.