I've personally sold on www.istockphoto.com your results will depend on the quality of photos that you take. You will typically get paid more money based on how popular your photos are, as well as if they are unique to that site or not. The most profitable photos are probably those taken on a white background for sure, as they are used on most blogs. These images are basically models posing with props or on their own in various ways that a blogger would feel fits in with the content of a given post. For this, I can't really give advice, just be as original as possible.