I’ve managed to add a custom wallpaper to the login wallpaper selection when I boot following the steps as detailed below:
I’m able to select and set the login background with this custom wallpaper every time I boot.
However if I simply log out of my account, the custom wallpaper appears briefly and is quickly replaced by the fallback wallpaper (the one with palm trees and cars). Clicking on the top right hand icon reveals no additional wallpapers to choose from (only the “Random” option).
Anyway that I can also set the wallpaper for login screen when I log out?