The Metropolitan Museum of Art has designated 375 000 public domain images as Creative Commons Zero (CC0), which means that those images can be freely used in any way whatsoever, regardless of jurisdiction. A simple ‘public domain’ designation in one location doesn’t necessarily mean that the item is considered public domain elsewhere. But a designation of CC0 is a clear worldwide public domain designation.
Those images include photos of some of the most famous paintings in the world. The Met will utilize a Wikimedian in Residence to expertly place those images into Wikimedia. Read at Wikimedia and at Creative Commons.
You can already use the public domain filter in the Met’s collection: http://www.metmuseum.org/art/collection#!?perPage=20&showOnly=openaccess.
Read more about that here: Met image resources and click on their organized collections!