How to customise GNOME default background and lock screen image?

your approach of copying the images to /usr/share/backgrounds/ and editing /usr/share/gnome-background-properties/ubuntu-wallpapers.xml will allow you to show them in Settings-->Bakcground-->Wallpapers and Settings-->Lock Screen-->Wallpapers

but the gsettings for these two are set to warty-final-ubuntu.png

$ gsettings get org.gnome.desktop.background picture-uri
'file:///usr/share/backgrounds/warty-final-ubuntu.png'
$ gsettings get org.gnome.desktop.screensaver picture-uri
'file:///usr/share/backgrounds/warty-final-ubuntu.png'
maduri@i7-4770U:~$ 

So, you need to compile these values after editing /usr/share/glib-2.0/schemas/10_ubuntu-settings.gschema.override file.

Run the command sudo nano /usr/share/glib-2.0/schemas/10_ubuntu-settings.gschema.override and edit the content for background and screensavers as below, changing to your actual image paths:

[org.gnome.desktop.background]
picture-uri = 'file:///usr/share/backgrounds/warty-final-ubuntu.png'

[org.gnome.desktop.screensaver]
picture-uri = 'file:///usr/share/backgrounds/warty-final-ubuntu.png'

Save the changes and run this command to compile the schemas:

sudo glib-compile-schemas /usr/share/glib-2.0/schemas/

Now you can check the gsettings value for these two wallpapers

gsettings get org.gnome.desktop.background picture-uri
gsettings get org.gnome.desktop.screensaver picture-uri

you are done. Restart the computer for the changes to take effect.

Please have a look of below Images which are self explanatory as I did it personally with Ubuntu 18.04.2 ISO with Ubuntu 18.04.2 as my OS

Booted with live USB and the resulting pictures are