Art typically enters into the public domain in 5 ways, but it's important to remember that even though an art object might be in the public domain, its reproduction may not be.
The Creative Commons is a set of licenses creators can use to release their work to the public. It allows for conditions for use and reuse, and there are several different types of CC licenses. Not all mean that something is freely available.
The 6 Types are:
Generally, CC-BY is the most permissive when searching for imagery.
When searching in museum websites, generally look for a tab that says “Collections” and search by date, place, or culture. You can search for things like “early Christian” or “byzantine,” or for objects like “lamp” or “amulet.”
Images need to be cited just as much as other works like books and journals.
The key features that you'll typically need are:
MLA Example: Ray, Man. Self-Portrait Assemblage. 1916. The Getty Museum, Los Angeles, CA. Jstor. Web. 24 August 2022.
APA Example: Mieris, F. (1661). Pictura [Online image]. Retrieved from http://www.ebscohost.com