image in mobile-only section does not show full height

Hi guys, i am working in Brine template. This is the URL: https://mauve-okra-yrme.squarespace.com/test-for-hero-module1. I am struggling really with this thing. I read a lot about it here and elsewhere but to no results:

I managed to add CSS code to show certain sections on desktop but not on mobile. And vice versa. This works fine. On my page, there is an Index with TEST in the name. My trouble is the image I use for the "New-Page". I can't get it to show well in retina, and all other mobile devices... I tried max-width, height, 100% etc etc... I am lost.... It must be a simple short piece of code...

I now added a 1500px wide image, ... don't worry about view on tablet for for ... I just need the full height to be shown.... also on retina...

ThanksThomas