In plasma 5.8 there is a switch to new lock screen. Being plain and simple it is nice indeed, but I'd like to have other background than solid black. I tried to change it in systemsettings -> Desktop Behaviour -> Sceeen Locking but with no success. No matter what image I select there, the background stays black. Reproducible: Always
Can't reproduce. Can you check whether your kscreenlocker is up to date? Changing it in System Settings → Workspace Behavior → Lock Screen → Wallpaper works for me.
I had probably some important bits missing. After upgrading to 5.8.0 from copr everything works fine. Closing the bug now.