I want to create an locked screen app for Windows Phone. I know how to create Apps but i don't know how to create an locked screen. 

For example, if a user opens my app, he can activate the locked screen wallpaper. Then the user can close the app, after 30 seconds the screen will lock(sleep stand of phone). On the locked screen there will be an image and information data that comes from the closed(tombstoned) app.

I checked lifecycle of a windows phone application but can not figure out how to build this?

Anyone got some articles or tutorials???