Stop Header Image From Cropping/Stretching

I’ve been banging my head against this for hours, scoured the internet to no end and cannot find a solution.

I wanted to make the header image area smaller, so I found a way to shrink its height, using some custom CSS and changing the code as “height:50vh;”

On my laptop browser, this approximates a height of about 266 pixels. I have a large image that I cropped using Gimp, to a size of 2000px width x 300 pixels height.

(For reference the recommended size for the WP Twenty Seventeen header image is 2000×1200.)

When I upload my 1200×300 size image using the customizer, when it is added to the media library there is a step that asks me to either ‘crop’ or ‘skip cropping’ the image. I select ‘skip cropping’ because I want the full image width of 1200 and height of 300, the actual image size. However, when I save, then refresh the website, the image is either cropped somehow, or else WordPress is zooming it to match the aspect ratio of the original image size (2000×1200).

How do I stop WP from doing this, so the header image shows at scale.

thanks