Hello, I don’t know how to change Plasma Login Manager’s wallpaper… In their GitHub/Invent (GitHub - KDE/plasma-login-manager: Plasma Login Manager · GitHub | Plasma / Plasma Login Manager · GitLab respectively) they say that, apparently (I’m not sure if that’s it) you can change the wallpaper via Kiosk… I found this page: Introduction to Kiosk | Developer but it didn’t help… in the same website (Documentation | Developer) I went to Wallpapers | Developer website… I finally thought I would help but the article is about a totally different thing.
TL;DR
I don’t know how to change Plasma Login wallpaper even after searching for it
For the time being it is not recommended to change the wallpaper on the Login Manager as Fedora/KDE Plasma have just cutover to using the Wayland Login Manager, instead of the Previous SDDM Manager. I would recommend waiting until KDE officially approves changes by the User. If you do and break it, the changes of somebody be able to help you fixing it are slim to none.
AlInAussieland
For the time being it is not recommended to change the wallpaper on the Login Manager as Fedora/KDE Plasma have just cutover to using the Wayland Login Manager, instead of the Previous SDDM Manager. I would recommend waiting until KDE officially approves changes by the User. If you do and break it, the changes of somebody be able to help you fixing it are slim to none.
Actually I found Login Screen section on System Settings and it’s possible to change the wallpaper from there, thanks for the response tho 
About breaking the system… it’s just a DM, I could reinstall it if it broke or using startplasma-wayland