GNU/Linux Desktop Survival Guide
by Graham Williams
20190313 The Gnome background can be set from the desktop with the right mouse button menu. Choose Change Background.
Shared background images are often available in /usr/share/backgrounds. The default background image for Ubuntu 18.04 is similarly located within this directory as /usr/share/backgrounds/warty-final-ubuntu.png:
REVIEW If you have a collection of background images in ~/.local/share/backgrounds then the following command can be used to randomly choose one to display. This could be put into a cron job to run each hour!
$ gconftool -t str -s /desktop/gnome/background/picture_filename \ "`find ~/.local/share/backgrounds | random | tail -n 1`"