Images of art, architecture, photographs and videos from art museums from around the world. Keyword search or browse by collection, type, or geography. Create an individual account to download the images. Downloaded images can be used in the classroom and non-commercial educational use.
Keyword search for images via Google, then to filter, click on Tools, and Usage Rights to limit to images with Creative Commons licensing. Easiest one to browse what website the image is coming from, to find ones from more official sources or institutions.
Search Flickr's large collection, but limited to those images with "no known copyright." However, it isn't always accurate. Double check the usage restrictions on each image by clicking on the image to see the description and licensing.