How do I customize desktop wallpaper slideshow?

I think I'm late but...

I created an xml background (with full paths descriptions), and I've saved it at /usr/share/backgrounds/my-background.xml

then I edited /usr/share/gnome-background-properties/ubuntu-wallpapers.xml adding

<wallpaper deleted="false">
 <name>My background</name>
 <filename>/usr/share/backgrounds/my-background.xml</filename>
 <options>zoom</options>
</wallpaper> 

just saves the file and the new slideshow wallpaper is shown in appearences window

hope it works for you :D

Also, when editing /usr/share/gnome-background-properties/ubuntu-wallpapers.xml, ommiting the options tags altogether will allow you to choose to either tile, zoom, center, scale, fill or span in the Appearance GUI in Settings.

Example using Locutus's example above:

<wallpaper deleted="false">
 <name>My background</name>
 <filename>/usr/share/backgrounds/my-background.xml</filename>
</wallpaper>