Columbia University Libraries/Information Services - Copyright Advisory Office
http://copyright.columbia.edu/copyright/fair-use/
Copyright Librarian
http://blog.lib.umn.edu/copyrightlibn/2012/05/free-and-legal-stuff-you-can-use.html
James Madison University's Free Media for Creative Use
http://www.lib.jmu.edu/research/freemedia.aspx
Penn State's Free Media Library
http://mediacommons.psu.edu/instruction-training/free-media-library/
Scholarly Communications @ Duke
http://blogs.library.duke.edu/scholcomm/
U.S. Government Summary of Copyright
http://www.copyright.gov/circs/circ01.pdf
Also, check out sites like Compfight and others mentioned in this Chronicle of Higher Education article to find images that have a Creative Commons license.