Banner thumbnail

Great theme btw.
I want to use a separate banner image per page.
I have this working by uploading ‘featured image’.
However, the image is very distorted.

Looking at the code
if ( has_post_thumbnail() ) {
$thumb_id = get_post_thumbnail_id();
$thumb_url_array = wp_get_attachment_image_src($thumb_id);
$thumb_url = $thumb_url_array[0];

Im not a programmer but know bits, (i have a child theme set)
Is there a way i can alter this to pull in the original image or not use a cropped thumbnail so the image looks better?

Thanks