Make a 1000px (or whatever width you’d like) image to use for the header, upload it to your media library, and copy down the image URL. Then replace YOUR_IMAGE_URL with the one for the image you uploaded and replace YOUR_IMAGE_HEIGHT with the height of that image.
Thank you for the help, I can see the image now stretching across the screen. If you dont mind, I have a follow up question. The image is just showing the top corner of the pic and not the whole image. How do you scale so that it fits?