Flickr has teamed up with Getty Images to sell your photos. Getty has a team of editors that look for photos on Flickr to sell through their network of buyers. Basically, when they find a photo they like, they will email you and ask if you want to list that photo for sale.
Flickr is a photo hosting site run by Yahoo! They have both free and pro accounts. The free account limits the amount of photos you can upload per month. The pro account does not.