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.
This online platform, which Google launched in 2011 in partnership with 17 museums (later expanded to more than 150), provides high-resolution images of artworks from around the world. You can also go on virtual tours of museums, find hidden connections between artworks (‘X Degrees of Separation’), and match your paint colors to your favorite work of art ― check out the Experiments page. It's also downloadable as a free app for mobile devices.
The art world’s answer to Wikipedia, this online art encyclopedia aims to make the world’s art “accessible to anyone and anywhere.” In addition to accessing images representing the works of thousands of artists (both public-domain and copyright-protected), you can learn what artist was born on this day, explore artistic movements (e.g. Post-Impressionism), and discover artists by nationality and time period.
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.