A single image can be uploaded to brand your site's public pages.
To Upload Image:
Go to Admin > Site Settings > Customize Your Site > Themes and Images > Page Banner Image then upload an image stored on your computer.
The banner is responsive (adjusts) to varying screen sizes and works best with images that are at least twice as wide as they are high.
The same image is used on all public pages, but is omitted in mobile views for easier navigation on very small screens.