When viewing your album as a Story, you will notice that your story background will be the first image in your album by default.
To change your Story background, select "Edit Background Image" to the left of your cover photo, or in the settings icon in the navigation bar.
You will see the image selector pop up. You can choose from any photo or animated gif in your account, or you may choose from one of our patterns:
Your newly selected background will show to anyone who views your Story.