Image header on mobile site

I have just about completed my website - however when checking it on a mobile phone or tablet the header image doesn't show although it looks great on my laptop. I'm not really that tech savvy but suspect it's something to do with the size of the image. I'm using the Keith theme and found that it says my theme works best with an image with a header size of 1200 × 280. I've tried re-sizing it to this size but it still doesn't show and then looks bad on the laptop. Any suggestions on how to sort this please??

You can set the header on mobile site on keith teme in: Appearance -> Customize -> Site logo

Bye!

Amazing ! Thank you very much! I spent hours trying to fix that - thank you for your help