The cover photo just refers to the main display photo that renters see first.
To change the main cover photo of a listing, go to the 'Edit Floorplan' option under the Properties tab on the bottom left-hand side of the Knock dashboard.
This will take you to a page where you can edit a floorplan's description and add/remove photos:
To change the cover photo, you'll want to drag and drop the pictures to re-sort them according to your desired order.
Once you're done, make sure to click Save! Now the cover photo has been changed!