Writers are lucky. They can describe photos and use them for reference without breaking copyright law. In fact, I've used flickr many times to add detail to descriptions I'm writing of both characters and locations. (With the geo-tagging feature, you can even find out what your old neighborhood looks like without going home.)
Visual artists have a rougher time of it. Copying a photograph too closely does break copyright law unless you get permission. However, Morgue File is a collection of donated pictures you can use for commercial projects and not have to worry about the long arm of the law. The only limitation is that they ask that you not sell prints of the photographs or represent them as your own work.