Ubuntu – How to span single wallpaper over dual monitors

First you need a wallpaper large enough for both monitors. You can either make one with the gimp or down load one. Alternately you can use a separate image for each monitor.

You can then install a handy little application, nitrogen

sudo apt-get install nitrogen

You then run nitrogen with the path to the directory with your picture(s)

nitrogen ~/Pictures

And select the image. At the bottom of nitrogen, select "automatic" and "Full screen" as options. alternately you can use nitrogen to set a separate image on each screen, up to you (sort of depends on your background images).