In this tip, Brad Tutterow focuses on one way in which you can update your application’s live tile background: The ShellTileSchedule.
Source: Code Badger
Of the two ways to update the background image, ShellTileSchedule is by far the easiest. The other mechanism is a push notification.
To start, you need to get the background image you will use for your background on a web server. ShellTileSchedule can only pull images off the web, not from the phone itself. This is one of the limitations of ShellTileSchedule. If you want to set background images to resources on the phone, look at using push notifications instead.