You can Search Harvard’s museum collections. They have scanned and photographed over 80,000 works of art that you can view online. Some of the scans are at very high resolution. If you play around with the URL you can get it to give you images up to 3000 pixels accross.
I’d suggest that you check the box that only shows record with images if you want to browse the art work. This will keep you from sorting through the items that haven’t been digitized yet.