Hey man, I don't wanna waste your time, but I just wanna follow up on this.
I was fiddling around on my profile, getting to know the interface, when I saw a setting for "promo-image". This was set to the exact picture I didn't like, the first one I uploaded... So appearantly, this image is auto-set when you FIRST set your cover image, but not updated when you change your cover image. Kind of a trap I would say, as you would expect the reverse, from reading the available tool-tip.
Just an fyi. Thanks, and see you out there :-)