Apparently someone will pay you more for a photo if they have exclusive usage right for a period of time, then if the same can be popping up elsewhere. So stock photo companies will want exclusive rights to your images. IOW they don't want you hawking the same photo on a dozen different outlets.
That said there are a number of sites getty, istockphoto, alamy, etc. Alamy has a neat calculator which will give you an industry standard price per photo usage:
http://www.alamy.com/price-calc.asp?...A-CF3207FF0DEB