I came across a new source for public domain imagery. As always, try to do a little research on the individual images to make sure sure someone isn't breaking copyright to upload them to the "public domain" sites to begin with.
Looks promising though!
Public Domain Pictures.net
If you know of any good ones to share, let me know, but first check the list I've made on the left hand side of this page to see what all is there already.
