I've been unable to find out what exactly is the specification of the parameter to be passed to the various org.gnome.desktop.background attributes. Particularly convoluted seems the picture-uri attribute. It seems to accept either the file:// URI of a file to set as the background image, or the file:// URI of an XML file describing a slideshow with rich properties. Those, however, I've not been able to find specified or documented anywhere. All I find are examples of varying quality. For example, what about the "transition" element and its attributes? Some examples mention a "type" attribute for which a valid value may be "fade". Where can I find a specification of all this?
For example, it is unclear how the transition is managed if the picture-uri is used to set an image directly. Can the transition only be controlled when using an XML file? Or can I also set the transition style and duration using some gsettings line?