Using stock images

Sorry that this question is so short and to the point, but does anyone happen to know of any tutorials or tips in general in using stock images? Any sort of stock images from old, torn paper type textures, or lighted objects, would be fairly useful. So far, I've found very few full icon tutorials that make use of stock pictuers, and because those are so specific it's not hard to make an icon that looks too similar to the example when following it. So I guess some general tips would be good.