The Library of Congress offers patrons this comprehensive database of out-of-copyright and public domain images.
This is a collection of sites that offer public domain images compiled by EdTechTeacher.org.
Unlike the Library of Congress which specializes in out-of-copyright images, Public Domain Pictures specializes in images that were never subject to copyright.
Customized for use in documents such as Word and PowerPoint, Microsoft hosts these royalty-free images for limited use.