The Creative Commons is a nonprofit organization that enables the sharing of images and other works by their creators. The Creative Commons allows creators to set the copyright conditions of their choice, including placing them in the public domain. A work may be used as long as the terms of the license that governs it is followed. See the Creative Commons Frequently Asked Questions page for details.
When looking for images that can be used freely, finding works that are in the public domain is very helpful as they can be freely used without seeking permission or paying royalties. Public domain is a term that applies to intellectual property that is not protected by copyright, either because the copyright term has expired or because the work was never under copyright protection. The United States Copyright Office states that "works in the public domain may be used freely without the permission of the former copyright owner." However, it is useful to cite or attribute the work, just as with any copyrighted work.