Custom Header Image

Hi

I read the instructions and watched the video on how to change the header in a customisable wp theme. However, when I load my own jpg it uploads it OK but then gives me a “crop” screen but the lines indicating where it will be cropped are too narrow – about 2/3rds the width of the original header. It will not allow me to increase the the size of the crop area.

I have edited my own jpg and made it the size of the original header so it should fit exactly.

I am trying to make this work one of my sites – which is not a completed site (I am just practising) https://thetrafficblog.com

Would appreciate any help please.
Thanks
Ray