I have just created a Java based little program/tool that creates the files needed for you to create a SlideShow background on Gnome using any pictures you want, my inspiration was this background: http://ubuntuforums.org/showthread.php?t=784881

you can download my Tool here: http://www.mediafire.com/download.php?hpwy6kuyhv2

I called it Shifty Background Generator. to run it you'll need the Java 6 JRE (or something that runs Java 6 compatible code)

unpack the .tar.gz wherever you like and then go to console and type:
Code:
java -jar /path/to/the/folder/SBGen.jar
then use the program (it's all in the console) and when you create the .xml drag it to [System -> preferences -> Appearance -> backgrounds tab] and drop it to install it.

minimize all windows and enjoy your slide show