Thanks @amymor!
Interesting how one (Microsoft) can make something so simple so complicated.

The two files (TranscodedWallpaper and CachedImage*.jpg) are not identical for me.
Which of these will ultimately be used for the display?
If I want to use a png or even bmp file, I can simply put it as "TranscodedWallpaper" in the mentioned folder?

Thanks and greetings, Martin