Modifying header for mobile viewing

Hi there! I’m modifying the CSS a little of your theme for a client. it’s been super easy. The only issue I’m having is with the header image. I tried uploading the header via the WP control panel, with the recommended dimensions, and it overlapped the nav menu, so I went into the actual css and did it myself and it worked just fine. The only issue is when viewing the site on a mobile device, the header image is cropped in half. I’ve gone through the CSS and even the responsive css, I can’t seem to find a way to modify it. I created my own @media query for it and it wouldn’t recognize it when publishing to the site. I’ve got to be missing something here! What should I modify so I can make the header image fit nicely in the mobile version of the site? Thanks! Great theme, well done!