dear bro most easy way to set wallpaper to extra desktop screen............

download cuteExplore

then open this cuteexplore

click on top where cuteexplore is showing
drop down menu select show hidden files...........

then go to
/home/user/.backgrounds

paste in this folder which wallpaper r u applying......

nd rename those wallpaper to background-5.png
background-6.png
background-7.png
background-8.png
background-9.png

hope ur desktop has been change....

goodluck...........