the only way I know to set wallpapers for Desktops >4 is to manually copy the wallpapers to /home/user/.backgrounds

and name them background-5.png, background-6.png and so on...
...which is sad, somehow...

this works even without installing also modded hildon-home.
Once I tried modded hildon home from http://my.arava.co.il/~matan/770/n90...+0m5_armel.deb
but this makes things worse! I can't use the whole "Desktop Option Set Background" version 0.3.58 has similar issues, but at least the menu opened....

hope there'll be a more handy way to set extra desktops with wallpapers any time soon....