Your Cover photo is the large beautiful image at the top of your Profile. You can change your Cover by clicking the "Edit your profile" button on the right side of your Profile. A pop-up window will appear where you can "Change your cover photo".
Cover images must be at least 2000x1000px resolution, though the bigger the better! Up to 5 preselected options are determined by photos that are of that resolution or larger, and sorted by most votes.
Note: You must click 'Save' in the bottom right corner for your new Cover Photo to be updated.