All the actual artwork of 19th century artists is in the public domain, however many photographs of the artwork are under copyright. There is a distinction between the original and a reproduction. While the original may be in public domain a reproduction can be under copyright.
Depending on how you intend to utilize the work you might be infringing the copyright of the photographer. The best thing is to work from photographs that have been released to the public domain, or from prints of the art that are old enough to be out from under copyright. How old they have to be varies depending on copyright jurisdiction.
As a practical matter, if you intend to redraw the images you have no worries, but if you are going to, say, make photoengraved images from photos it’s best to be sure the photo you’re working from is not under copyright.
Check Wikimedia Commons for images. Those are all under one or another of the Creative Commons licenses.