Many Flickr users have chosen to protect their work with a Creative Commons license, and you can browse or search through photos under each type of license.
Yeah, f'real, as in 127,100 pictures available under attribution license alone. You can also use the search, so you could, say, find all pictures tagged with "snowboard" with the attribution license. OK. That's the bag of chips right there. I mean, sure you may have to wade through a lot of fill before you find that one perfect picture for your next school report, but that's not much downside.
And don't forget the mother-search: the CC search engine, where you can also find other sorts of media. Nice demo of RDF in action as well. See:
Thinking XML: The commonsof creativity, Uche Ogbuji, developerWorks, May 2003