Different Header Media for mobiles in portrait mode

I am working on www.emmaelizabethspmu.co.uk for a client. The Header Media displays fine on a desktop and in landscape mode on my mobile, but as you can see does not in portrait on a mobile. What is the best way around this please? Is it possible to have an alternative image display JUST in portrait mode on a mobile? If so, could you advise the CSS coding please? Thanks, Chris