Front page sections do not show up

Hello everyone

I use the Zerif lite theme because I like the appearence of it, eventough I do not focus on the ability to concentrate all the info on one page. For that reason, I have never needed the front page sections.
For some time now, I wanted to add a Background image tough. As the inbuilt function of Zerif would look quite pretty, I tried to use the big title section.
Here comes the problem, tough: When I add a background image it doesn’t show up. Even if I activate the green or red button, there is no sign of a background image.
I hope someone can tell me, what I did wrong. I don’t really have the ressources or patience to build up everything from scratch.
My website is http://whatsalp.org .
Thanks in advance