Angel indeed saw oil paintings in the Nightmare Realm's Hanging Jail Stairway. However, all these paintings were concentrated at the topmost floor, which is the Warden's residence.
The Warden of the Hanging Jail Stairway seems to be a person who greatly loves oil paintings, as every corridor in his residence is lined with numerous artworks. Each of these artworks is an exquisite masterpiece.
Initially, when Angel left the Hanging Jail Stairway, he even took two paintings from the Warden's gallery to bring back to reality.
To this day, those two paintings still lie inside Angel's bracelet.
The oil painting on the wall of this ruin is naturally not one of those two that Angel brought out, but he had seen similar paintings in the Warden's gallery.
In other words, this painting very likely originated from the top of the Hanging Jail Stairway, the Warden's gallery.
