The Artstor Digital Library (library.artstor.org) is a resource containing over 2 million images from the world's museums, archives, libraries, scholars, and artists. Use Artstor to find images for papers, presentations, and study in the humanities. There's also a set of tools for sharing images, curating groups of images, downloading them directly into powerpoint presentations, and comparing and contrasting images.
This guide is adapted from the Artstor Libguides page. Further information can be found there.