Artstor is the first place you should start when looking for images.
With the Advanced Search you can limit your search by:
You can find countless images here, but use with caution and respect the owners' copyright (yes, things on the open Web are still under copyright). Go to "search tools" and find images that are labeled for noncommercial use; you can also specify large-sized images if you need a high resolution.
In February 2017, the Metropolitan Museum of Art flung wide its open-access doors. Now some 375,000 images at this New York institution are available through a Creative Commons Zero license. Limit your search to “Public Domain Artworks” for images you can download and use freely.