From Wikimedia Commons, the free media repository
These are generally licensed as CC-BY, with a few exceptions being released into the Public Domain.
Kathedraal van Antwerpen
Dom of Trier
Abbey of St. Matthias (Trier)
St. Willibrord Basilika (Echternach)
- Not my work, but PD-Old stuff I uploaded. Listing it here to keep track of what I uploaded.