The header image should be at least 2048x512 pixels (ratio 4:1) and the most important element must be in the middle. The image needs to be that big in order to appear correct in both horizontal and portrait view. When viewed in landscape (such as on a laptop screen), most of the image will be visible. When viewed in portrait (such as on a smartphone), the outer left and right edges may be cropped. If you imagine your image was cut into four even columns, the middle two columns are where you should position your most important visual element.