The Public Domain Photo Database is a great collection of thousands of public domain photos for commercial and private use. A very few have limited use restrictions, note.

‘Just keep in mind that if you put photos here you lose control of how they can be used. People can and will be making money from them, and they won’t be giving you a cut. The benefit for me is that I’m enriching the public domain and giving corporate greed a poke in the eye with a sharp stick.’