Not limited to art but for cultural heritage collections in general.

There is the OpenGLAM Survey, a list of Galleres, Libraries, Archives, and Museum sharing their collections under open licenses: https://docs.google.com/spreadsheets/d/1yc8z0z7XdhVKvhTbD2_z...

Then there are also aggregators like dp.la, cultural.jp, digitalnz.org, europeana.eu etc that might also be of interest.