Here is a guide that shows you how to assign a wallpaper to a specific workspace. To do this you need to have the CompizConfig Settings Manager (ccsm) and the extra compiz plugins installed.
You can assign the images using the Wallpaper plugin from the CompizConfig Settings Manager (ccsm). To start ccsm open a terminal window press Alt+F2 and type: ccsm and follow these steps:
1. Click on the Utility button.
2. Click on the Wallpaper button.
3. Click on the New button.
4. Click on the browse button and select the image.
5. Change the position of the image to your likings.
6. Enable Wallpaper plugin (your panels will crash).
7. Close ccsm
8. Press Alt+F2 and type gconf-editor.
9. Click on apps -> nautilus -> preferences
10. Scroll down until you see: show_desktop and disable it then close the configuration editor.
When done log out and back in again and the wallpapers are set and your panels will look normal again.
There is also an option to Rotate the wallpapers, to do this enable Cycle Wallpapers.
Please note that when using this on Ubuntu Unity the Alt+F2 function (run a command) only works when your pointer is on a panel. When the pointer is on your desktop you will not be able to type a command after pressing Alt+F2. On Ubuntu Classic it all works like it’s supposed to work.
If you don’t have the CompizConfig Settings Manager installed open a terminal window (press Alt+F2 and type: gnome-terminal) and copy+paste the following line:
sudo apt-get install compizconfig-settings-manager compiz-plugins-extra