By default, the background in X Window is a grey background. Some window managers have their own
options to set other backgrounds, and some don't.
A program you can use to set your background is a program that comes with most Linux distributions. It's
called XV, and you can specify which background image you want with this line:xv -quit -root image.gif
This will set the background to be the image.gif file. You can add that line in your .xinitrc file, in your home
directory, so it is executed each time you launch X Window.
"Windows Linux Tips: Background image in X Window"
Reference : techpraveen.com