We recommend a square logo for your business logo, as this has shown to be best for mobile devices. This is not forced, but recommended.
Your cover image will automatically be cropped to fill up all the space in the header.
The space in the header depends on your screen size and device orientation. So its best to use something like a landscape, something abstract or anything that wont look bad when cropped at the top or sides.
It is also possible to leave this blank and uncluttered.