I found the following over at Fotostoria. At the end of World War II, Hitler’s Ministry of Public Information and Propaganda order to produce photographic records of art works that couldn’t be moved to secure locations to avoid them being damaged during the war. This lead to photographs being taken of important buildings (churches, castles, …) and their interiors, using Agfa’s colour slide technology. Since the slides started to physically deteriorate, at the end of the 1990s, efforts were made to digitize and preserve them. The digital archive is publicly available.
Unfortunately, it’s only in German, but it’s well worth the visit anyway. To navigate, click on “Ortssuche” (which will get you here) or “Kuenstlersuche” (which will get you here). The former will let you browse by location, the latter by the name of the original artist.