Ok, firstly you must make sure that you have those three conditions:
1. You have to use Ubuntu 8.04 or 7.10
2. You have to install and have CCSM. Check under System ->Preferences if Advanced Desktop Effect Setting is there.
3. For more better result I recommend Avant Windows Navigator and cairo dock. You can install it easily by clicking those link that I gave you.
After you completed those conditions, this is the tutorial and the how-to install the different wallpaper on each workspaces:
1. Go to System -> Preferences -> Advanced Desktop Settings
2. Select Desktop Cube ->Appearance -> Background images
3. Click New under background images and add all the images you want as background.
4. Press Alt+F2 and type this code: gconf-editor and then simply hit enter
5. Go under apps -> nautilus -> preference and deselect the “Show Desktop”
Some screenshots (I’m using a screenshots from other site, I will use mine as soon as possible):
Well.. As simple as that, you already get the different wallpapers on each workspaces.
N.B: You will see that you cannot right-click on the desktop and your desktop shortcut will gone, but don’t worry because you still can access all of your original desktop in Places -> Desktop directory. And if you still insists for the shortcut, the cairo dock or avant windows navigator will replace the shortcut with more cool way . You must sacrifice your shortcut if you want to use this different wallpaper on each workspaces but I already gave you some other method to replace it right? I will post the method screenshots as soon as possible.