Where can I find free stock photography?
First, please take a look at UA's page on copyright information.
That said, here are some sites you can use to find free stock photography and illustrations:
Flickr Creative Commons: www.flickr.com/creativecommons/
Wikimedia Commons: http://commons.wikimedia.org/wiki/Main_Page
Flickr Commons: http://www.flickr.com/commons (most of these photos have no known copyright restrictions)
Google Images: http://www.google.com/images (use Advanced Search to limit your results to items labeled for reuse)
Morgue File: www.morguefile.com
Stock.xchng: http://www.sxc.hu (requires a free account)
It is important to be aware of the licensing and copyright information. While some of these images can be freely used and edited without any sort of attribution, some cannot be modified, and some must have attribution.
For more copyright information and tips on using Google Images and Flickr, check out this guide from Duke University.