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).
rdenny
tradeprint1
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
https://www.si.edu/openaccess