There are tons of images available online for educational use. When you use images that you did not create, make sure that you are following copyright guidelines.
This video is quick overview of the Creative Commons.
"Public domain" images are not protected by copyright. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission. A couple examples of websites that share images that are in the public domain are listed below. For more information about the public domain click here.