SOFTWARE/OS VERSIONS OpenSUSE Tumbleweed Kernel Version 6.3.4-1 Plasma Version 5.27.5 KDE frameworks Version 5.106.0 Qt Version 5.15.9 The option to change your wallpaper on desktop, SSDM, and lockscreen should be accessible from the appearance tab in the system settings under the global theme option. Currently to change the desktop wallpaper you have to right click on the desktop and select 'configure desktop and wallpaper'. To change the lockscreen wallpaper you have to click 'workspace behavior' -> 'screen locking'-> 'configure appearance'. For SSDM you have to click 'startup and shutdown' -> 'login screen' -> click image. It would be nice if the options to change the wallpapers for the desktop, SSDM, and lockscreen were available in the 'appearance' section on the 'System Settings' program.
It's in progress; see https://phabricator.kde.org/T12622. Since we already have that open to track the work, I'm going to close this bug report so we don't end up working the conversation into different places.