Well suck it up, but I wasted a few hours trying to find a way to change the background of my desktop. KDE used to work, but after the last update it stopped working.

Now there are suggestions what to make:
https://sourceforge.net/projects/mateslideshow/files/stats/timeline,

http://www.fandigital.com/2012/06/change-theme-command-line-gnome.html

but no in Debian, in Debian one has to use:

#!/usr/bin/zsh

# Set your folder with the backgrounds
bgfolder="$HOME/Bilder/NaturBilder"

while true
do
 pic=$(ls $bgfolder/*.jpg | shuf -n1)
 gsettings set org.mate.background picture-filename $pic
 sleep 3m
done

gsettings is the magic incarnation. At least now it changes here every 3 minutes. I guess I would have to waste more time on figuring out how to get different pictures on 2 screens:

Now because I hated it so much during my study times,-  now I drive you mad also with it: „The rest is left as an exercise for you“…

Gross -stuff, how we spend hours figuring out things. Must be a programmers problem 😉