In the "Configure desktop", it would be nice to have a button to apply the wallpaper to all the screens. Now one needs to open the configuration dialog multiple times to change the wallpaper on each screen.
Looks related to https://bugs.kde.org/show_bug.cgi?id=367666
Duplicate of the bug I submitted on year before :-) *** This bug has been marked as a duplicate of bug 367666 ***