Backgrounds are helpful because they make it easier to look at the screen for a long time. However, it's important to make sure backgrounds aren't distracting. Backgrounds must loop perfectly or the transition will be jarring.
When importing your backgrounds, make sure to choose the "Background" preset instead of "Media". This automatically sets the background to fill the screen. If the background is a video, it also makes sure the volume is set to 0 and that it loops.
To set a background for a segment, click the gear icon on the right. In the panel that appears, click the button labeled "Background". Choose an image or video from your library, and it will be set as the background. This only takes effect on the displays when the slide is changed.
Finding Good Backgrounds
Floodlight has a few motion backgrounds available to download for free here.