How do I set a full width header image on BP Corporate and make it responsive/adaptive

it has been decided for me that the header image of this new site has to be a full width image !!
I have installed a dummy image to see the effect. Unfortunately it doesnt work as a responsive header. It maintains its fixed size as the site changes depending on what you view it on. I am sure this is an easy fix, but I just dont know how.

The temp sandbox url is :-

Hope somebody can help, I would be so pleased.

Many thanks and kindest regards