What's the difference between public domain and royalty free images?

Question

If there is any difference at all.

Answer 1

Essentially, "public domain" means that nobody owns the image - any copyright on it has expired or the owner has formally stated that the work is now in the public domain.  You can use it without any restrictions.

"Royalty free" means that there is an owner, but that owner has stated that no payment is necessary in order to use it.  Usually you have to get their permission, though, and/or give credit to them as the owner or creator.