I have no experience selling through stock agencies, but as an art director, I purchased quite a lot. Enough that the ad agency had a special discounted rate from Getty and Corbis. My understanding is the stock agency gets the lion's share of the sale and to make decent money, one has to have a lot of photos listed with them. There are different rates, depending on usage. Run-of-the mill landscapes or cityscapes are quite common and probably not in much demand. Any photo with a recognizable person must have a model release, too. Many factors apply to the cost of a photo, from reproduction size, quantity and market type. An image that will appear in mass media will get more $ than if it runs in a short-run promotional flyer in a small market. And then there is royalty-free. Basically, dirt-cheap rates and the image can be used in any manner except on an item that is being resold, like a T-shirt or mug. As far as I know, for all stock photos the photographer retains their original copyright (not sure about royalty-free) but the agency usually has exclusive rights to sell the image. That is, if a client sees the photo on the stock house website and then contacts the photographer directly, the photographer is bound to send them to the stock agency for the transaction.