Have you ever meet these problems?
I just tested to try and recreate the issue. When I first set a picture it took affect right away, however when i changed it, it wouldn’t change until I logged out and back in.
However I didn’t get any error messages when trying to change it. Any chance you could translate the error message for me?
In the first picture? “Error occured when saving changes”
This happens when you choose a picture which is not in the waiting list.
Fixed by update to KDE 5.78 and Plasma 5.21
In fact, this bug may disappear after update and happen after next update.