Windows 10 background automatically switches to other pictures?

In between, my PC simply changes my background image and not to any one I have on my PC, but to one that my colleague has on his laptop.

I don't even have this picture myself. It is the other way around, if I then change my picture again at my request, it also changes after a few days to the one I have chosen, which he does not have.

How can that be? There's actually no connection between the computers except that his laptop was once in my network.

And you are not connected to the same Microsoft account? That's exactly what happens…

Not that I know of. I set up my PC myself and he set up his.
Where exactly would this option be?

Right click on "Customize" on the desktop. Here you see on the far right under "related settings" the item "synchronize settings". Click on it.

Here is e.g. "Synchronize the… Using XYZ." this should be YOUR Microsoft account (or none if you don't use it).

Otherwise, the sync can also be turned off completely here.

Insane, there was actually his laptop with it as a device. And I'm getting desperate.