When creating a new story, you'll see a pre-selected color as a banner. To change the banner, first click on it, then select the '...' and select either 'edit' or 'remove'.

If you click on 'edit', you'll be able to upload an image from your device, select an image from our library of high resolution images, add a link to an image found on the web or upload from Google Drive. If you upload your own, the recommended dimensions are 1920x400px or higher.

If you remove a banner, you can get it back as on the screenshot below: