I’ve just stumbled across, that the feature for putting several pictures in a row and cycle them through as a wallpaper is not in the current version of Gnome for Ubuntu. I found a really nice tutorial at gnome-look.org where it’s described very well and easy going.
At the bottom you can adjust the time for each picture and the time for the crossfading.
To do a slideshow just:
add this to your repositories with the software manager:
and install “crebs” through synaptic afterwards or do following in the terminal:
sudo add-apt-repository ppa:crebs/ppa sudo apt-get update sudo apt-get install crebs