The image you upload must be at least 399px wide–one that’s at least 720px wide will work best. You can upload a photo already cropped and resized to precisely those sizes. Or you can upload a larger image, in which case you’ll be given a chance to move the image to choose the crop you’d like displayed.
You can only designate one photo as your Cover Photo. Panoramas are ideal. Simple crops also work.
And don’t forget to center image content so it displays well on mobile!