Often instructors are looking for images to use in their courses because images can liven up the course and help students understand the course material.
A picture is worth a thousand words, but it might also be worth a thousand dollars if your school gets hit with a copyright violation claim. —Eric Curts
There are many high quality pictures that can be used without any licensing concerns. These can include images that are released under creative commons, or are in the public domain, or simply are copyright-free.
Eric Curts compiled a list of free image sites and tools for schools that you may find helpful in your search for free images.
Additionally, you may find an image you want to use, but you would like to make changes to it. You can find image editing software suggestions in the Xavier University Library Digital Humanities Toolbox. Just make sure the image copyright gives you permission to modify the image.
ICYMI, read my blog post on Digital Copyrights for copyright information.