There are several museum websites, library databases, and books which are good sources of art images. You will be able to find images for consultation as well as for reuse in course work, criticism, and other academic uses. Consult the guide on finding and using images for more information.
Search with the Library catalogue or FindMore using an artist or art historical subject as a search term. Try including the terms "exhibition catalogue" or "catalogue raisonne" to include annotated catalogué which often include images.