Museum websites are excellent sources for images. Search specific museum websites to locate collections for your project. You should also check Flickr and Artstor as many museums and cultural institutions are using these platforms to share images.
According to copyright.gov "Copyright, a form of intellectual property law, protects original works of authorship including literary, dramatic, musical, and artistic works, such as poetry, novels, movies, songs, computer software, and architecture."
How do I know if something is still under copyright? There is a chart for that, created by Cornell University.
To learn more about Creative Commons licenses watch Creative Commons Kiwi by Creative Commons Aotearoa New Zealand.You might also explore the CC website, see who uses CC licenses, or try building your own Creative Commons license.