Hi There,
Also I have same problem too. I have already reinstalled modified-hildon-desktop and tried as below

>gconftool -s /apps/osso/hildon-desktop/views/active -t list --list-type int [1,2,3,4,5,6,7]

for i in 1 2 3 4 5 6 7 ; do gconftool -s /apps/osso/hildon-desktop/views/$i/bg-image -t /home/users/MyDocs/$i.jpg ; done

existing 4 desktop background has been updated but 5,6 and 7 still black