The image displayed in the theme by default on the landing page (homepage), below the menu, is not the configured header image but a banner image placeholder, which is configurable from the theme’s options under Landing Page > Slider.
The sample image is not visible in the customizer panel. To replace it simply assign a different image. To remove the sample image assign any other image in its place and then remove it.
Thanks for your reply, but that’s what I tried to do and it didn’t work… It showed the new picture in the sidebar but it just wouldn’t take the old one out.
I now had to use another theme for the time being, but I might be able to test it on another website.