This is really simple and easy to do, and there are a huge number of ways to do it. This is my method, using feh.

  1. First, install feh:

    sudo apt-get install feh
  2. Second, configure the wallpaper to be set on startup. For this, we’ll need the ~/.xsessionrc file. Place this line somewhere in .xsessionrc, after #!/bin/bash but before the if statements:

    # Set background image with feh
    feh --bg-scale /usr/share/backgrounds/warty-final-ubuntu.png &

This just invokes feh and tells it which image to set as the background. I have the default warty background here, but you can change the file path to any image you want.