Got a reply, finally, in the forum.

Hey there tassitus,
If you have a child theme installed, you could set the height of your header with some CSS, which might help your cropping issues… However that can have implications for the responsive design and mobile-toggle…
Another idea would be to hide the background image (via CSS) on mobile…
I’m not sure I have the ideal work-around for you in this case though.

On a different note: I didn’t think I was all that dumb, but there are times when I doubt my mental capacities LOL … When you’ve made a blockquote, and want to go on typing afterwards; how do you get out of it? Is going into HTML-editing my only option? That’s what I did now, because I couldn’t find any other way.