Public Domain Images
Copyright.gov explains public domain images: The public domain is not a place. A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.
Creative Commons Images
Images that are not in the Public Domain are protected under copyright. Some copyrighted images may be offered for reuse under a Creative Commons license which grants permission to the public to access, share and reproduce those works. There are several different Creative Commons licenses that may be applied, so be sure to check what uses the license allows.
For more information on Public Domain & Creative Commons Images see the Harvard Law School Library's guide on Finding Public Domain & Creative Commons Media.