Robin Good has compiled a list of places that provide royalty free images.
" Finding quality images and photos for complementing an important article, essay or news report is already quite a challenge for many. Imagine when the goal is not just too find good images, but find some that you could openly and freely use without needing to pay royalties or one-time publishing rights to someone."
"PDPhoto.org is a repository for free public domain photos. Unless something is clearly marked as being copyrighted, you can assume it is free to use. But if you intend to use an image you find here for commercial use, please be aware that standards for such use are higher. Specifically, you should assume no model release was obtained. And pictures featuring products or property should be used with care. The photos are here to be used, but I don't want you to get either of us in trouble over it."