I'd have to agree with Wizzo. Don't fuck around with so called "public domain" pics.
If you do, it will just be a matter of time before you get a letter from a lawyer.
Look at what just happened to the guy who runs Funky Nipples:
http://ss5.sexshare.com/~funkynipples/
He got off easy.
And there is something else to consider, besides copyright issues.
How can you prove the people in the pictures are of legal age? You can't, unless you are posting granny pics or something.