The "Fair Use" clause of the U.S. Copyright Act allows for a legal defense again copyright infringement claims.

Learn more  about Fair Use from the US Copyright office. Teaching and learning are typically considered to be protected uses of materials, but it is important to properly attribute images and other works. 

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 licensed 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.