I had the same problem took me about one hour to find out what to do. But this worked for me :

1) follow these instructions : http://maemobriefs.blogspot.com/2010...e-screens.html

2) make sure the package rootsh is installed and run the following commands in xterm:

sudo gainroot

cd /home/user/MyDocs

wget http:// my.arava.co.il/~matan/770/n900/hildon-home_0.3.58+0m5_armel.deb

dpkg -i hildon-home_0.3.58+0m5_armel.deb

3) in xterm :

gconftool-2 -s /apps/osso/hildon-desktop/views/5/bg-image -t string /home/user/MyDocs/.images/NAMEOFYOURIMAGE.jpg

you can change the number of the desktop you want the image attached to by changing the 5 into a 6, a 7 , a 3 or whatever. F.e.

for changing desktop nr 6. :

gconftool-2 -s /apps/osso/hildon-desktop/views/6/bg-image -t string /home/user/MyDocs/.images/NAMEOFYOURIMAGE.jpg

hope this works for you!