As a visual medium, art historical research has benefited from the availability of high quality digital image databases. Some of the most popular include:
Art museum websites are an excellent place to find images of the artworks in their collections, or comparative images for other works of art you may be studying. Often, a museum may have digitized a substantial part of their collection and placed the images online.
Here are several large, useful collections (with image numbers as of July 2017, when available):