sudo apt-get update
sudo apt-get install crebs
Select your photos and name your slideshow.
After creating it. Crebs store it as xml file in $HOME/.crebs
Then rename & replace crebs xml file to background-1.xml in the path /usr/share/backgrounds/contestRight mouse click on Desktop and select: Change Desktop background
Select your slideshow in the appearance window.
If for some reason the crebs repo doesn't work, you can always edit your /usr/share/backgrounds/contest/background-1.xml and change the images path for those which you want to use.
Don't forget to assign proper rights to the xml file. I use to set 666 with chmod command. (chmod 666 background-1.xml)
Enjoy it