Sometimes it is necessary to grab a quick photo or bit of art off of a clip-art or public domain site. This site, Public Domain Photos, has some photos free to use. Granted, they are not spectacular, but their terms and conditions beat the other sites which pretend to display art in the public domain, but do not actually allow free use of it (a strange turn of events, to try and place copyright restrictions on the public domain ~ really! Call your site what it is, folks. If it requires royalties or other permissions, it's really not in the public domain, is it?)
This seems to be a site that truly offers Photos in the Public Domain.