The "Fair Use" clause of the U.S. Copyright Act allows you to use copyrighted material, including images, in an academic/classroom setting, without obtaining explicit permission as long as the image is properly attributed.
If you want to use images for non-academic purposes, you may need to either get written permission from the copyright holder to do so, or limit yourself to images that have been licensed by the creator for re-use. In almost all cases, you will still need to attribute the image to its original creator. You can find such images at the following places:
Creative Commons: Search for images that have been licenced for re-use.
Google or Google Image Advanced Search: Advanced Search allows you to limit your results to images that allow for legal re-use.