Thanks, no need for login with this issue. I can see how you need to style the image just by visiting the link you posted.
The header image is a background image, applied to the #header div element. And so its fluidity needs to be set via the background-size property on the Background property group. Your theme already sets the background-size to cover on mobile devices, which is what you want. But the padding-bottom of 280px is a bit too much on mobile. Setting it to around 90px should reveal more of the image.
I see you have already created a custom selector in Microthemer. If you click its name in the top toolbar you can change the selector to #header. And then set the bottom padding to 90px via the Phone tab. You can remove the other styles you added.
Or, you can delete your current selector and create a new one in targeting mode by clicking on the header image.