The image should have a resolution of at least 2048x512 pixels (i. e. an aspect ratio of 4:1) and should show the main subject as close to the center as possible.
The picture should be so wide because it should be displayed correctly on screens in landscape and portrait format. If it is displayed in landscape format (e. g. on a computer screen), it usually appears (almost) in full width. If it is displayed in portrait format (e. g. on a smartphone), the quarters are cut off at the left and right edges.
Imagine the picture is cut vertically into four equally sized pieces. The essential motif should be in the second and third quarters of the picture.