Public Domain Archive hosts photos that are entirely free of rights whatsoever. You are allowed to download them freely and do whatever you’d like to do. Use them in any kind of project, no matter how much money you earn with it. You need not even attribute or backlink or give the original creator any kind of back reference.

However, if you need a certain photo fast, look elsewhere. But if you like to browse and get inspired, the Public Domain Archive might be the perfect place for you. In any case add the service to your bookmark collection.