This is a (quickly made, minimally tested) Perl script to generate a GNOME slideshow background from a directory of images.
It takes as input the path to the folder of images, then makes the background-1.xml file and installs the whole thing to /usr/share/backgrounds. You'll have to manually choose the background-1.xml file from the Add... button on the GNOME background changer dialog. Defaults are 5-minute showings with 5-second transitions.

Usage: ./mkslidebg.pl /path/to/images/folder