As we said earlier, we find a mistake on that line, which will be amended in the next version and we are extremely sorry for the wrong information that we have mistakenly provided to you.
and to show the full wider banner there are other solutions too, but using widget kit will allow the functionality to show slideshow as well as single image. it can be shown on selected pages, whereas header image will be show on every pages, you will not be able show/hide header image on any specific pages plus you will only be able to use 1 image for the banner in the whole site. So we recommend you to use widget kit and if you have any questions regarding it we will provide you the support.
Still you want just one image on the whole site as banner image please mail me at [email protected] i will forward you a file that will show the header image with 500px height.