Copyright "is a form of protection grounded in the U.S. Constitution and granted by law for original works of authorship fixed in a tangible medium of expression. Copyright covers both published and unpublished works." Learn more here.
Fair Use allows "the unlicensed use of copyright-protected works in certain circumstances." Learn more here.
Creative Commons licenses make creative works "freely available for legal use, sharing, repurposing, and remixing" by "any member of the public." The CC Search search engine lets you "search for free content in the public domain and under Creative Commons licenses. Learn more about CC licenses here."
Quick guides to citing images:
College Art Association Guidelines regarding Fair Use and Copyright:
Statement on Fair Use from the Visual Resources Association