“I would like to have the sticky header with a background image (my header image) instead of the choosen color in the customizer. Do you have an additional code for this?”
You could add a header image in ‘Customize’ > ‘Header Image’
and then to make the image only appear when scrolling down the page add this ‘Additional CSS’:
#masthead:not(.scrolled) {
background-image: none !important;
}