You have eight templates to choose from for the Cover Photo look: Cosmos, Reef, Bondi, West, Oakwood, Classic, Edge, and Anchor. You can see a preview of how these templates look on your cover photo under the Change Cover Photo tab by simply clicking on each one.
You can change the cover photo for your Collection in 2 ways:
1. Through the Design tab in the Collection Settings (also accessed by clicking on "Change Cover") by selecting Change photo (as shown above). This allows you to upload a brand new photo, directly to the Cover Photo.
2. By selecting one of the images you have already uploaded to the Collection. Simply click the image you'd like to use as the Cover Photo, and then use the "More" button to select "Make Cover":
What is the ratio/dimension of the Cover Photo?