The Artstor Digital Library offers 3 million images from leading museums, photo archives, scientists and artists, including many rare and important collections that are not available anywhere else. It is a valuable source of visual data for teaching and educational purposes.
You can find a list of collections here: https://www.artstor.org/collections/.
For more information see the Getting Started with Artstor Digital Library
MU has free trial access to the Artstore until December 31, 2021.