ubuntu wallpaper unable to display

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http://askubuntu.com – Ubuntu 13.04 default wallpaper directory is "/usr/share/backgrounds/", Is this correct? I wrote a code that can add pictures to the directory and create "contest/raring.xml". But it's does not work. the wallpaper directory has many pictures but i can not choice any picture to wallpaper. "Right-click desktop" -> "Change Desktop Background" ,there only one black wallpaper! (HowTos)