I have dual monitor set up in Ubuntu 11.10. I want to have a single wallpaper spanned across both monitors. How do I do this?
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)
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).
Nitrogen answer works, but another tweak is necessary. Run Advanced Settings in Gnome Tweak Tool, go to Desktop section and switch Have file manager handle the desktop off. Then wallpapers set via Nitrogen will be displayed.
If you do not have the Gnome Tweak Tool, you can use this command instead. It's an easier solution, too:
gsettings set org.gnome.desktop.background show-desktop-icons false
However there is strong disadvantage. All icons disappear from the desktop and the context menu can't be invoked. Unless you need icons on desktop this solution may be useful for you.
I do not appear to be able to add a comment to an answer due to my low rep (lolz), so this is meant to be on @topr's answer.
I am the author of Nitrogen and there is code in there to detect a gnome desktop and set it properly so that you don't need to make this change, but it appears to not be functioning on modern Ubuntus. I will investigate.
For Ubuntu 16.04 and higher versions which don't offer a 'span' option in the settings or no options at all, you can run the following command to force a span setting:
gsettings set org.gnome.desktop.background picture-options spanned
The effect is instant.
I've tried this with 18.04, but it seems like it should work with 16.04 and up.
you can use shotwell to resize te image as your combined monotors resolution then set it as background using the default image viewer of ubuntu.