I did post "may vary on different Mint DE's though" if you saw it
. You could try KDE as well, you may end up liking it better.
I try to keep info around for when I help users here, but Im not always successful. Anyway it is said that you can do this with compiz, but at the cost of desktop icons. Compiz needs to have the the wallpaper plugin installed which probably means a later version than what some users may have? You can check out this guys video at youtube (which is not real good) but may show you some tips. http://www.youtube.com/watch?v=vsrfXzRC1F0
Also I have read before that there were several Dev's working on this Gnome problem in the background. This feature that y'all are after is one of the most highly requested features that Gnome users ask for. The only reference in my info folder that was left about this subject was this: http://gsocblog.jsharpe.net/
Im sorry, I havent had time today to research this for you so this is all that I have on hand ATM.
Google gnome desktops+multiple wallpapers
That should get you some good hits guys.
If I find anything else I will try to post it asap.