The following sites contain a number of free and legal images to use in school projects. Since free and legal images are more scarce than just searching Google Images, you may have to do more hunting than you are used to, but I guess that's the price of respecting other people's art!

http://www.morguefile.com/
This is a pretty good site, with a lot of images.  After doing a search make sure not to go into the "Dreamstime" section.  Images in that section cost money.

http://www.stockvault.net/
This is another a pretty good site, with a lot of images.  Just like with Morguefile.com, make sure not to go into the "Dreamstime" section.  Images in that section cost money.

http://www.flickr.com/creativecommons/
Make sure that you stay in the "Creative Commons" section of Flickr.  Those are the only images that are legal to use on your projects.  Also, pay attention to if you need to give credit to the photographer.

For those of you doing research on water, this website may be helpful for you.  It's all about the "hidden" water we use.  

If you like studying battle strategies, you should check out this website called Civil War Animated that outlines all the formations and tactics of both sides in a moving map format.