With 22.04 now out as a stable release, it would be great to get this added! It looks like a trivial change on top of the impish support but impish support was removed without a useful comment in the commit.

Yes, I was wondering about that. I do have a PR open, it was against the impish commit, which has been reverted since.

I have a branch that works against Ubuntu 22.04 if you want, https://github.com/mendhak/change-gdm-background/tree/ubuntu-jammy but would be great to get some insight from @thiggy01 !

There's also an answer on StackOverflow showing how to change the wallpaper without relying on this script. I did test it on Ubuntu 22.04 and 20.04 just now, seems ok.

First, sudo apt install systemd-container

Then you can change the wallpaper using:

sudo machinectl shell gdm@ /bin/bash -c "gsettings set com.ubuntu.login-screen background-picture-uri 'file:///usr/share/backgrounds/warty-final-ubuntu.png'"