This page lists recommended collections of images with licenses allowing use in your work. Be sure to read and comply with the specific license for each image you find.
Public Domain
If an image is in the public domain, you may use it without permission of a copyright holder.
Creative Commons Licenses
Images with Creative Commons licenses allow certain types of re-use without asking permission from the copyright holder.
The 6 different Creative Commons licenses tell you what you can do for each type of license. For example, some CC licenses allow you to remix and use commercially, and others don't.
Attribution
When you use free images like these, it's still a good idea to provide attribution, even if it's not required.