Comment 10 for bug 1970081

Me too, noticed this problem after upgrading from ubuntu 20.04 to ubuntu 22.04

The background picture I had selected in menu -> control centre -> Login Window is not the one shown, instead I see the jellyfish with green background picture

The display manager in use is called lightdm

The configuration files are under /etc/lighdm/

/etc/lightdm/
-- lightdm.conf
-- lightdm.conf.d/
-- slick-greeter.conf
-- users.conf

The background image setting is in slick-greeter.conf, I have the following values:
[Greeter]
background=/home/ubuntu/Pictures/mars.jpg
activate-numlock=true
show-a11y=false
show-power=false

Seems that the settings under /etc/lightdm/lightdm.conf.d/* take precedence, in fact, as an experiment I did

$ cd /etc/lightdm
$ sudo mv lightdm.conf.d lightdm.conf.d.old

after rebooting, now I can see the mars.jpg picture as the background of the Login Window as expected