I've sent an email to the folks at Public Domain Pictures http://www.publicdomainpictures.net/ because they seem to be conflicted in their own distribution of images. I found a nice pic I wanted to redistribute but found the follow, seemingly contradictory, text:

"License: Public Domain [http://creativecommons.org/publicdomain/zero/1.0/]. If you are going to redistribute this image online, a hyperlink to this particular page is mandatory."

Seems to me they don't actually understand what "Public Domain" really means...