Big Sur wallpapers location
There was a question like mine in here: https://discussions.apple.com/thread/252326166 but it was marked as solved, but there is no solution in that thread.
Now I worked on the same issue and solved it by adding the "old" location in "/Library/Desktop Pictures" and moved my pictures to that location. Now my pictures appear in the Desktop Prefs.
Mac mini, macOS 11.6