Ghillies is right, nearly all photos, on the net, or otherwise, are protected by copyright, which belongs to the photographer, who will have the original file or negative, and be able to prove ownership. Image "theft" occurs daily, most photographers live with it, but would get a bit pissed off if they found their photos being used commercially. If you see a photo you would like to use you must, at the very least, make an effort to find out who took it and contact them before using it.
Goes against the grain to say it, but you can find very cheap images on the microstock sites (ooh wash