@fireflywebs: Hello there,
This is difficult to achieve because when you have text in the header image position is used to align the text container with the header image and the header image size in smaller screens depends on the amount of text over the image.
What you can try doing is crop the whole header image section on your homepage like in this screenshot. Then upload the image as the header image and see how it looks in the smaller screens.
Let me know if this works out!
Kind Regards,
Skandha