Artstor is a non-profit organization that offers a digital library of more than 2 million high-quality images for education and research.
You can search by keyword or collections, and you can also search among the thousands of images in the Images for Academic Publishing (IAP) program. IAP makes available publication-quality images for use in scholarly publications free of charge.
Credo Reference is a customizable general reference solution for learners and librarians. Offering more than 475 highly-regarded titles from over 70 publishers, Credo covers every major subject from the world's best publishers of reference.
On-line encyclopedia, with full text articles covering every aspect of the visual arts from prehistory to the present day. It also contains art images and links to museums and galleries, bibliographies, and biographies.
Public Domain Images
Public domain images are images, including photos, clip arts, and vectors, whose copyright has expired or never existed.These images can be used for free by everyone without the need of attributing them, and they can be used for both personal and commercial uses.
On February 7, 2017, The Metropolitan Museum of Art implemented a new policy known as Open Access, which makes images of artworks it believes to be in the public domain widely and freely available for unrestricted use, and at no cost, in accordance with the Creative Commons Zero (CC0).
See the link for more info about searching for and identifying these images.
While not all images on Google Images are Creative Commons licensed, it is possible to limit your search results to only images available under a Creative Commons license. Google does not use the exact same language of Creative Commons, but you can still find reusable images.