Ubuntu – Change Unity wallpaper in Ubuntu 18.04

In previous Ubuntu releases, the user was able to modify the Unity wallpaper in the System Settings programme (Appearance panel). This action no longer has an effect on Ubuntu 18.04.

How can I change the Unity wallpaper in this Ubuntu release?

Update I: this is the output requested by Joshua Besneatte:

$ pstree -s $(pidof X)|grep gdm
        |-gdm3-+-gdm-session-wor-+-gdm-wayland-ses-+-gnome-session-b-+-gnome-shell-+-Xwayland---14*[{Xwayland}]
        |      |                 |                 `-2*[{gdm-wayland-ses}]
        |      |                 `-2*[{gdm-session-wor}]
        |      |-gdm-session-wor-+-gdm-x-session-+-Xorg---3*[{Xorg}]
        |      |                 |               `-2*[{gdm-x-session}]
        |      |                 `-2*[{gdm-session-wor}]
        |      `-2*[{gdm3}]

Update II: following some discussions on Google+ I tried disabling Wayland. This had no effect, though.

Update III: a similar question was opened in September of 2019. Please mark that other as duplicate, not this one.