Typically, no photo, or artwork for that matter, will be public domain for many years after its creation. Copyright used to expire a number of years after the creators death. I don't know where that lies anymore, with the estates being able to retain copyright (which was never the intention of the law), it just gets ugly.
That said, there are royalty free image available. Some work on a subscription basis. There are a couple of good ones that offer free copyright and royalty free images. The first is http://www.gimp-savvy.com, which has acres of free to use images. The second is http://sxc.hu which is a community of artists and photographers providing images for artistic use.
"Channeling is just bad ventriloquism. You use another voice, but people can see your lips moving." -- Penn Jillette