On Startup and Shutdown, I changed the wallpaper to use for the startup/login screen. However, the change doesn't hold. On reboot, it's back to that rainbow picture that came with the OS. Reproducible: Always Steps to Reproduce: 1. change background image for login screen under Startup and Shutdown 2. reboot 3. observe that no change took place
I have the same issues on 5.6 and 5.7 - the reason I think is because when you select an image from your own home filesystem SDDM can't reach it, picking a file in /usr/shr and it happily accept the change. The best way for me have always been renaming the image you want to use and moving it manually to the SDDM theme you select and renaming it background.png (thereby removing the standard background.png)
For me this is using Arch based distro so it is distro independent
same problem using kde neon
*** This bug has been marked as a duplicate of bug 344867 ***