Creative Commons, the public interest group that supports a re-use and fair-use friendly copyright license structure, has created a new search engine, http://search.creativecommons.org/ which makes it very easy to search for copy friendly images across several large image sites, including Google Images, Flickr, Yahoo!, blip.tv and others.
This allows you to be sure that the images you retrieve in your searches have been marked by their authors under a Creative Commons Copyright license, which means you can use them in your classes, courses, and research, without having to worry about infringement.