A single image can be uploaded to brand your site's public pages.
To Upload Image:
Go to Site Settings > Customize Your Site > Look & Feel > Page Banner.
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 optimum banner size (to minimize image re-sizing across the entire spectrum of screen sizes) is 1030 px wide x 120 px high.
The same image is used on all public pages, but is omitted in mobile views for easier navigation on very small screens.