How to disable changing desktop background (wallpapers) automatically in Ubuntu 12.04?

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http://askubuntu.com – After I upgraded to the Ubuntu 12.04, it seems if I want a wallpaper, the only option is the default one, and the desktop background (wallpapers) would be changed throughout the day (once per hour). The problem is that it seems to be not very stable, especially in the case of using two monitors, it crashes during changing the wallpaper from time to time, quite annoying. Is there a way to disable changing desktop background (wallpapers) throughout the day in Ubuntu 12.04? Many thanks, Jibo (HowTos)