Individuals are widely responsible for not only finding the images used in their published works but also for securing copyright permission to use them. Here is the Library & IT guide to copyright information along with other copyright information.
Here are the online collections of some of the biggest museums that provide public domain images of some of their collections, if the work itself is in the public domain (or assumed to be in the public domain if the creator is unknown).
Remember that for many of these collections, there are copyrighted images as well. Many of these sites provide search tools or indicators of if an image is in the public domain. Want to be sure? Reach out to your librarian for support navigating these sites!