Header Elements place the image as a CSS background image.
The default option is for that image to Cover the content area, which means some of it may get cropped. To get around this we use % ( top and bottom ) padding to make it responsive.
Try this:
Calculate the original images aspect ratio: H / W * 100
e.g 1080 / 1920 * 100 = 56%
Now set the header element to full width
And split the aspect ratio % between top and bottom padding e.g 28% top and 28% bottom.