PDPhoto is a database of photos that are in the public domain and can be used for any purpose.
From the site:
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.