Aloha, when i first lnstalled cachyOS, i had a background image on the desktop and login screen that was Scarlet tree. It seemed to have been part of the breeze theme. Later on things changed and the image i very much liked was gone. I found plasma wallpapers containing it and was able to get it back as background image. But not for login screen etc. I guess it is because the theme / settings are now somewhat inconsistent. Is there a way to revert back to the theme containing the image? Or any other way to have Scarlet Tree set as system default wallpaper / login screen?
If Scarlet Tree is installed, you can also change the SDDM background.
You just need to know in which folder the image is located.
Thanks, your reply made me look again. I realized there is a small icon directly at the image shown in the settings for SDDM, which allows to change the background image … 
Sorry for the rough answer.
I only realized afterwards that Scarlet Tree is the old SDDM.
So you probably weren’t faced with a change of scenery yet and had to find this option first.