This is an article from Education World. It has a list of copyright-friendly image searches.
Ideal for searching images by educational category.
Pics4Learning should be one of the first places you and your students look for photos and other images online. Educators and students have donated thousands of images for Pics4Learning, so anything you find is free for use in the classroom. Pics4Learning also encourages you and your class to donate digital pictures. (No photos of minors or faces of adults). The site even includes a collaborative project with a frog named Jacques who documents his visits across the world through photos. Don't miss it!
With a goal of 5 million items -- from speeches to images to video to digitized text and more -- the American Memory project is by far the best source for digital content specific to American History. More than 100 thematic collections make it easy to find images specific to your class's needs. The Browse feature allows you to look for images based on U.S. regions, media type (photos? maps?), time period, or topic. Be sure to click the Teacher link on the home page to find excellent tips for using this Web site in your classroom. The teacher-based resources include a monthly chat, guiding questions to teach students to respond to the images they find, lesson plans, interactive games, and much more.