This should do it. First go to appearance > header and delete the header you have uploaded there (set it back to original). Next upload the header you want to use to the media library. Do not use the header uploader as it will always crop to the original size.
Put the URL of that newly uploaded image into the background declaration where it says URL OF IMAGE between the double quote marks. Adjust the height value to match your image. The border radius line below is to fix a mistake in the original RTL CSS.