Fixed backgrounds are available for web prototypes only. This allows you to fix the background while the forefront screen scrolls vertically (parallax-type scrolling).
To set your fixed background:
1. Go to your prototype.
2. Go into Build mode.
3. Click the configuration at the bottom right corner of the screen next to the lightning bolt.
4. Click Upload New under Background.
5. Click the lock icon.
Be sure to save your changes.You can also view this InVision prototype and this screenshot to help guide you to the proper setup.