Public Domain
"A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner." (US Copyright Office)
Many US federal documents and images are not covered by copyright and are in the public domain. There are other public domain sources from international sources, libraries, museums, and other sources.
Creative Commons (CC)
You will need some basic information to cite images that you find. A free or public domain images also needs to be cited.
You should note the following information if possible:
I encourage you to use Zotero to save information about images you find. Most likely, you will use the "Artwork" category for these images.
List of large image collections. Many of these collections include public domain or Creative Commons license images. However, for each images you use, check the copyright information provided by the organization and/or website. You will always need to cite your sources whether they are under copyright or not.
Portals
Institutions