I am running Juno and I have a 3-monitor-setup with two in portrait and one in landscape.
Display setting shows this layout:
before I change my layout using
xrandr commands (3 monitors in landscape), I can change wallpaper by right clicking on it and do set to background, or just use app like Fondo or Variety.
After display setting takes effect, nothing above works any more.
Loki failed on this one when it was just released but fixed it after many system updates.
io.elementary.contract.set-wallpaper failed. I ran it on command line didn't work either. gnome setting doesn't work either.
what I did was:
/usr/lib/x86_64-linux-gnu/io.elementary.contract.set-wallpaper '///usr/share/backgrounds/Ashim DSilva.jpg'
system log did not issue any error message and on terminal there was no feedback at all.