You need to change the landing page banner image. For this go to the Customizer >> Landing Page >> Slider and choose a Banner Image.
This did the trick! Thank you very much.
I have one more problem regarding the menu: I would like to get rid of the side bar menu (the one you get when you click on the 3 stripes on the top right side of the page). I only want to menu that shows up on the top of the page and has the names of the pages. Is there any way to do this?