After you are logged in you will have access to your profile page. You can get to your profile by clicking the menu item located under the Members Only tab called My Profile. You will now see your profile page which includes new menu items. You will see a menu item called Change Cover Image, click it. Now you will see an upload section which will allow you to upload an image for the cover photo.
The cover photo is the large photo used at the top of your profile page. This photo is publicly visible to all members.
The recommended size for your cover photo is 972px wide, and 315px tall. Using a smaller photo will result in a tiled effect of the image.