I don’t know what you are referring to by “image in the main menu”.
If you are referring to the image displayed under the menu on the theme’s landing page feature (the homepage by default), this is configurable from the theme’s options under Landing Page > Slider.
Due to a WordPress limitation, 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.