Copyright Friendly is a great wiki site that has links to tons of copyright friendly images and sound for use in projects, web pages, and blogs.
The site is easy to navigate and the listings tell you what each site contains. For most of them, you do not have to cite them for educational purposes, but you should at least give credit to them. Many sites have copyright notices on them, so you should read the terms of use of the images.
For more information on the Creative Commons Licenses that most sites use these days, visit there SITE.
Here are some more posts on copyrights for teachers: