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 Media section of your account. After that, you'll simply go into your live stream and hit the Edit button. From there you'll scroll down the About tab to where it says Offline Media, and all of the videos in your account will show up as options to choose from. Hit Apply, and you're all set.
Note: Features such as offline pictures and videos, and pre roll videos only will work if you are using Stream Monkey's player. Anywhere you are sending the m3u8 link (apps, set top box channels), it will not show up.