Select the settings widget from the button of the menu on the left hand side. Then select "edit creator page."
From there, scroll down until you see "cover image." Make sure to use a photo that will work stretched out (a resolution of 1600x400 works best).
If you'd like to remove your cover photo, you will need to select a replacement photo. You can hover over the current image to add a new photo.
Note: We're slowly rolling out an improved way to edit your creator page. If you are one of the lucky users to be testing out the new feature, you can upload your cover photo directly from your creator page by hovering over the top of your page.