I am building a Custom ISO of Ubuntu 18.04 LTS using CUBIC and I'd like to change the default background and lock screen image.
I added the backgrounds in
/usr/share/backgrounds, changed the background in
/usr/share/gnome-background-properties/ubuntu-wallpapers.xml file, but it does not seems to affect new users (for example the OEM user).
Also, I did not find any way to change the lock screen image.