When you're using any kind of media for a class project (whether it be images, audio, or video), your best bet is to use open content. Content that is considered “open” is that which is available under an open license or part of the Public Domain.
Openly licensed content, as the name implies, is any work that has an open license assigned to it. This means that the rightsholder has given advanced permission for anyone to use and reuse their work (subject to the license terms). When you use open content, you do not need to restrict access to your class or academic environment.
The most common type of open license is the Creative Commons (CC) License. There are 6 different CC licenses, each with their own terms (the Creative Commons website breaks down each of these in plain English). When you come across a work with a CC license, you are free to reuse it, as long as you attribute the original creator. Note that most of the CC licenses have additional conditions, so it’s important to read the specific license terms.
Media that is in the Public Domain (not restricted by copyright) is also safe to use in class projects. A work becomes part of the Public Domain when its copyright expires, its copyright was not properly renewed, or the creator dedicates the image to the Public Domain. There are many resources that curate collections of Public Domain materials (see sections below on finding open content), so you don’t have to make these determinations yourself.
Always read and comply with the use restrictions for specific sources.
Always cite or attribute images someone else created.
Most images and other creative works created by the U.S. Government are part of the Public Domain (with a few exceptions).