Does anyone know?

I would say check out iStockphoto, they have a good reputation, but I must say you have to have many more pictures to make an impact. Otherwise you could join a site like associated content where you can post your pictures and get paid for views. It means you keep on earning for the same picture every month. Check out my profile for link and payment proof

There are quite a few, actually: shutterpoint, fotolia, iStockPhoto, just to name a few. Be aware that photos of people tend to sell better than natural or abstract ones.