I found THE solution! (thanks to Ali for his information in comment #10)
I had the same problem. It appears to be that Ubuntu seeks for XML entrys in ALL the files in /usr/share/gnome-backgroud-properties directory, not only the xml files (you can even have a file named my-wallpapers.ogg but filled as XML with your pictures entrys and it will work). So it is not necesary to add the entrys in precise-wallpapers.xml file or in ubuntu-wallpapers.xml file, you can have your own file like my-wallpapers.xml. - Now, why the pictures appears twice in the appearance menue? It's because text editors like gedit creates backup files for every file you are changing (for my-wallpapers.xml it will create my- wallpapers.xml~). All you have to do is to remove all the backup files and you will have the appearance menu just like you wanted it. PD: Text editors backup files are hidden by default, because of that one can no realize why the pictures appears twice. you can remove the backup files with sudo rm, or you can run nautilus as root and hit the "view hidden files" option. -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to gnome-control-center in Ubuntu. https://bugs.launchpad.net/bugs/934047 Title: Double wallpaper-entries in appearance menu Status in “gnome-control-center” package in Ubuntu: Confirmed Bug description: I've added a wallpaper to the /usr/share/backgrounds folder and added an entry for it in the /usr/share/gnome-background-properties/ubuntu- wallpapers.xml file. Since then, every wallpaper appears twice in the appearance menue. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/934047/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : [email protected] Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp

