There are numerous tools you can use to search online for public domain images or images licensed for easy re-use. Here are a few options with information on how to use them. If you find something you wish to use, please be sure to make a note of the specific license attached to it if there is one. Many of the images relevant to this class may come from digitized library collections, but they will also pull up other images provided by people around the world.
Start by performing a normal image search in Google:
In the search results, click on “Tools” under the search bar. One of the filter options that appears will be for “usage rights.” The options allow you to limit results to images to either "creative commons licenses" or "commercial & other licenses"—although either option should be fine for this class since you are unlikely to be modifying the original, and you are making a noncommercial use.
When you find an image you are interested in, click on it. Use the “View Image” link on the right to go to a full size version of the image you can download. Use the “Visit” link in the same place to go to the page where this image is included: you must do this to find the specific license information attached to the image in the context of the page. You should probably keep a link to the image and the license attached to it in your notes for when you use the image.