There are two types of stock photo sites. One type deals with professional photographers and sell photos to producers of books and publications. I am a featured aquatic photographer with one of those sites (acclaim photography) and with a very small amount of photos there (due ONLY to my own lazy nature of not uploading more) I have made some real money. But It is not easy to connect with these people, they want phtoographers who have been published already in trade publications, which I have a lot of experience with. they contract the companies and take huge commissions (usually 50%) but also guarantee your image rights and make sure that the contracts are performed properly.
The other type are sites that sell photos for $5-10 and I would never put any of my material on those sites. The quality is questionable, and I would not trust them with my images. At that price, you are just not getting the right clientele to buy your images.