Using XML animated background creator to create auto-changing wallpapers is a snip.
It allows you to specify the folder and images you wish to use, the duration of each wallpaper and auto-generates the required .xml file for GNOME to read from. The wallpapers are shown in alphabetical order, so bear this in mind if you want to choose the order of rotation.
As with other .xml wallpapers – including the set Ubuntu ships with by default – you can manually ‘skip’ wallpapers by using the controls in Appearance Properties.
To use the creator you can either place the ‘xmlback’ script in the directory of wallpapers you wish to use and run it. This will prompt you to specify duration, create the .xml file and then set it as your wallpaper.
You can also install this creator in nautilus by running the ‘install.sh’ script. This method allows you to go to apply the script whever you so via right-click > scripts > xmlback.
If you’re creative you can find man clever ways to use this script such as simulating a morning to night effect by setting appropriately scened images to auto-change at hourly intervals.
You can download XML Animated Background Generator @ Gnome-Look