Following up on yesterdays' post, there's perhaps an even better way to find yourself some free stock imagery. Everystockphoto is a search engine that pulls its images from a range of online imagebanks, all free of course. Amongst them are sites like Stock.xchng and flickr. It even utilizes OpenSearch to make it even more useful.
The downside of using EveryStockPhoto however, is that you can't instantly be sure of the liscensing permissions of the images, and you have to look into it yourself, which can take a little longer than simply searching on places like Stock.xchng. Plus, the search results are often full of amateur photography results pulled from flickr (most of which are tagged in some stupid way for a stock repository) But, where there's volume, there's more of a chance that your obscure search results will dredge up something.