FreeFoto is a site with over 130,000 images, organized into 3638 categories that are free to use online. It is easy to navigate with some great images that you can use for free. Schools can also download the web-sized images to use offline in school projects. The images are great quality.
The only thing the site requires is that you link back to the site and give them credit for the image.
If you are looking for images to use in class or for your students to use, check out FreeFoto. http://www.freefoto.com/index.jsp
Pics4Learning - copyright free pictures for education
Free Clip Art from Discovery Education