How to adjust the header image to mobile screen

Hello team Tasman

Can you please help me on how to adjust the header image to fit well on a mobile phone screen, as you can see the moment we resize the browser to the size of a mobile screen, the header image also resizes (which is a good thing), but in my case black spaces started appearing on top and below of the header image. It would be nice if the black spaces does not appear.

Thank you.

The page I need help with: [log in to see the link]