http://search.creativecommons.org/
Many sites allow one to search for media that have been somehow labeled as freely available for reuse. This tool brings those services together. While this service is not a guarantee that image is available for reuse, it does provide a filtering service.
Classroom Idea: To model the need for conscious thought when reusing someone else's work, send students here for media searches. I think Flickr, especially, with the clickable license link, is a great model and teacher of respecting others creative work.
