Your image must be a jpeg file with a maximum width of 3240 pixels, and a maximum height of 2160 pixels.   Images larger than that will be downsized to conform.

Images with a width less than 1200 pixels or a height of less than 750 pixels will not be allowed.  The rules are relaxed in the case of a "letterbox" or "column" image image, so if an image has a width of greater than 2200 pixels, the height is ignored.  If a portrait image has a height equal to 2160 pixels, the width is ignored.