Ubuntu 22.04 LTS
Disable guest account in Ubuntu Mate 22.04
sudo nano /etc/lightdm/lightdm.conf.d/91-arctica-greeter-guest-session.conf
change the following to false:
allow-guest=false
greeter-allow-guest=false
Change the login screen background:
Rename the original image and name the image to be the background image with the file name that of the original image. Seems the image has to have an PNG extension.
Location: /usr/share/backgrounds/ubuntu-mate-jammy
Name of login background image on my system: Jammy-Jellyfish_WP_4096x2304_Green.png