In legalese speak, public domain images are creative or original works not protected under intellectual property laws.
They are images whose copyrights have expired, or were never copyrighted. In other words, a public domain image assumes that the creator of the work has relinquished his work to the public or can no longer claim exclusive rights over it.
The work is now owned by the public.
Once an image enters the public domain, anyone can copy, manipulate, distribute, display or simply use it anyway he wants without legal encumbrances.