Public Domain
A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone.
There are sites devoted to providing images that are in public domain. Additionally, all works created by the U.S. government are in the public domain.
For more information on Public Domain, go here.
Creative Commons
"Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools" (Creative Commons website). By using images from the Creative Commons, it allows you to use images for free with some restrictions such as giving credit (attribution) to the original creator.
For more information on Creative Commons, go here.