The image displayed by the theme by default on the landing page (homepage), under the menu, is not the configured header image but a static slider placeholder, which is configurable from the theme’s options under Landing Page > Slider.
Due to technical limitations, 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 that one.
The static slider buttons are configurable from the same Landing Page options panel.