Sorry cradom.
I did manage to find Trackpad (by clicking in the Search icon in the top right corner and writing "Trackpad", did open a menu where shows "trackpad", I did click on it and did open).
But it doesn't look at all like yours and the commands on it are totally different. To start with comes as standard ticked ""Use two fingers to scroll" (as in many laptops). But there is a command already ticked that says "For secondary clicks, place two fingers on the trackpad then click the button". I did it and, of course, opens "Change Desktop Background".
By the way, in my top left corner doesn't exist "System Preferences", is clicking in the Apple icon. I did follow your idea of making a folder for wallpapers. Thank you.
Have a nice day.
Tino