Ubuntu 18.04 with Gnome Desktop
please note: this is copied straight from Ubuntu documentation, it's worth a look :)
You can change the image used for your backgrounds or set it to be a solid color.
Open the Activities overview and start typing Background.
Click on Background to open the panel.
Select the Background or Lock Screen.
There are three choices displayed on top:
Select Wallpapers to use one of the many professional background
images that ship with GNOME. Some wallpapers change throughout the
day. These wallpapers have a small clock icon in the bottom-right
Select Pictures to use one of your own photos from your Pictures
folder. Most photo management applications store photos there. If you
would like to use an image that is not in your Pictures folder, either
use Files by right-clicking on the image file and selecting Set as
Wallpaper, or Image Viewer by opening the image file, clicking the
menu button in the titlebar and selecting Set as Wallpaper.
Select Colors to just use a flat color.
The settings are applied immediately.
When done, switch to an empty workspace to view your entire desktop.