Anytime you are offline (not sending a stream), the player will show an image that says "Stream Offline" by default. If you'd like, you can replace that image with a video that will play instead.
To set up an offline video, you'll need to save/upload a video into the File Manager. After that, you'll simply go into the stream you'd like to set the offline video on, and hit Options. From there you'll hit the menu under Offline Video, and all of the videos in your File Manager will show up as options to choose from. Hit save, and you're all set.