The new Windows 8.1 option "Show my desktop background on Start" is not available on my system. Actually, it was until I had to change my motherboard (and had a lot a crashes). Now that my PC is running normally again, that option is greyed and therefore unavailable. All the other options on the tab are functional.
I tried with other backgrounds to no avail. I don't know where to start digging.
I just reactivated my windows 8, because of the Mobo change (2*48 numbers to copy/enter on your phone, I love U Microsoft), and now the problem is gone. One more mystery of Computers life. Thanks for your support anyway.
So it sounds like the Win 8.1 OS was not yet activated after the MoBo change. Thanks for posting your solution. This might be helpful to others reading this.