ARTstor is a database of images from museums, artists, libraries, colleges and universities, scholars, private collections, and photo archives available for teaching, education, and scholarship, with all images cleared for educational use.
Creative Commons is an organization that creates licenses for people who would like to make their content shareable and reusable. Content with a CC license often requires the re-user to credit the creator. Openverse is a search tool that allows you to find CC-licensed works.