PDPhoto is a Free Public Domain Photo Database 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 there 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 there to be used, but they don't want you to get either party in trouble over it.