The ideal image ratio should be 3:2, for example 660x440 pixel or 1500x1000 pixel. The file should be a jpg, png or gif.
If the uploaded image ratio is within 20% of the 3:2 ratio, for example 680x440, it will be cropped to the target ratio (660x440) to avoid gray bars on the sides.
If the image is not even close to the target ratio (e.g. more than 20% should be cropped), for example 1200x440, no cropping will happen at all.