Hi Francois Zietsman,
Please add this css on child theme style.css file to make background image responsive:
#pref-top {
background-size: 100% 100% !important;
}
Note: Making background image responsive may distort images on mobile devices
Note: Please add this css on child theme style.css file , so your changes will not lost on theme update.
Let us know how it will goes.
Thanks