Art does not reproduce the visible, rather it makes visible -- Paul Klee
As a visual medium, art historical research has benefited from online sources of high quality digital image databases. Some of the most popular include:
In addition to academic databases, both public and private sources are making digital content available online.
Art museums' online collections websites are an excellent place to find images of the artworks they own, or comparative images for other works of art you may be studying.
Here are several large, useful collections (with image numbers as of July 2017):
Intended to protect the creative output of authors and other artists, copyright is an evolving acknowledgement of ownership. As new methods of expression are developed, and new ways to transmit information grow in acceptance, determining what can be copyrighted and by whom is no longer as straightforward as it was at the doctrine's inception, when it was applied only to "maps, charts, and books."
A full discussion of copyright is beyond the scope of this guide, but it should be noted that simply because you can download an image does not mean you are free to use it without limitation; the object itself may have no copyright protection, but its image may be licensed by an institution or photographer. Generally speaking, fair use exceptions are made for certain purposes, including academic research, but it is the user's responsibility to understand any potential copyright issues.