Don't forget to give credit for pictures taken off the Web. Many of these images are subject to copyright. DO NOT CITE Google as the place you found your picture. Google is a search engine that organizes images for you. It does not create them!
Free Photos offers public domain and free stock images that are able to be previewed and downloaded (and used in presentations, handouts, etc.) at no cost.
Search Flickr, Yahoo!, Google, OWL Music Search, SpinXpress, and BlipTV to "find Creative Commons-licensed media that you can legally share and reuse for free"