Header image is assigned to show in the background. You add whatever size image, it will cover the whole area. To make sure image will not cut off you need to crop and make it’s height and width exactly same as your header height and width. For cropping, you can use any tools like Photoshop or you can crop it online.
Hi, Add the below CSS to your child theme style.css
display: block !important;
flex: 0 0 100%;
And make sure child theme is activated. Otherwise changes will not be reflected. You can add this code to Appearance -> Customize -> Additional CSS.