A friend asked about online repositories of photos that can be used for academic presentations and I mentioned several sources. I realized after sending the list that many others may not know about the wealth of resources available, especially for teaching and learning.
Online photo repositories (check usage rights for each):
Free images for your inspiration, reference and use in your creative work, be it commercial or not! http://morguefile.com/
Creative Commons search, but check license of each photo: http://search.creativecommons.org/
Commons project from various institutions, including The Library of Congress: http://www.flickr.com/commons/
Wikimedia Commons, with many public domain images: http://commons.wikimedia.org/wiki/Main_Page
Most of the media in these collections are attached to generous copyright licensing. http://copyrightfriendly.wikispaces.com/