In the United States a photograph of a public domain 2D image is still also public domain if the photograph is considered a faithful reproduction of the public domain 2D image: https://en.wikipedia.org/wiki/Bridgeman_Art_Library_v._Corel....

However, I think the laws for this vary quite a bit across countries, and in many countries the photograph is considered a new copyright work. In general it's pretty frustrating how many legal barriers there are to accessing and reusing old works of art. Thankfully a growing number of museums have made things easy with clear copyright releases (Rijksmuseum, Paris Musées, the MET), but others seem more interested in preserving their ability to sell prints.