Im writing this as a working draft. I followed this JumpCloud Guide; however, it no longer seems to work. Particularly on macOS Big Sur.
The way I made it happen was using JumpCloud MDM Profiles and iMazing Profile Editor to generate the profile.
I had to send the wallpaper file using a command and then move it to a place thats not /tmp/ so it didnt get removed.
mv /tmp/wallpaper.jpg /Users/shared/wallpaper.jpg
Then in the iMazing profile editor I had to set the location (the screenshot was an earlier screenie) I then uploaded this as a custom MDM policy inside JumpCloud, applied it to a group of devices and that was that!
As usual, test on a device first and I take no responsibility for anything it does or doesnt do to your systems.