The big problem with Artvee remains that they don’t link back to the image source, which would prove whether it’s properly public domain or not. As it stands you have to take them at their word.
IANAL (and I am linking to Wikipedia) but I don't believe this is a problem in the US (as long as the painting is out of copywrite) due to Bridgeman Art Library v. Corel Corp. https://en.m.wikipedia.org/wiki/Bridgeman_Art_Library_v._Cor... . In Germany on the other hand, things are different: https://news.ycombinator.com/item?id=11983043
This would be solved by linking each photo back to the originating gallery.
(I contribute to Standard Ebooks – https://standardebooks.org/ – and would like to use Artvee’s imagery for covers, but without a link back to the originating site to check for a CC0 grant it ends up being a trawl through Hathi looking for photographic proof that the image is out of copyright.)