The Met Museum has shared many of their paintings under CC0 - meaning that if the artist has been dead for 90+ years - we can use the image. They are all pretty high res. If you do find an image you plan on using (bulletin, cover, banner, anything), please make sure to list the credit byline properly (ie: The Trojan Women Setting Fire to Their Fleet - Claude Lorrain - ca. 1643 - Image Courtesy of The Metropolitan Museum of Art).

https://www.artrenewal.org/MyARC/Home/Index

rdenny
tradeprint1

Here is a good list of the top sources of Public Domain on the web https://www.moma.co.uk/public-domain-images/

Getty has public domain images too https://search.getty.edu/gateway/search?q=&cat=highlight&f=%22Open+Content+Images%22&rows=10&srt=a&dir=s&pg=1

It is called Open Content Images