From page 204 onwards, of Apple's iOS Human Interface Guidelines document,
To enhance the user’s experience at app launch, you must provide at
least one launch image. A launch image looks very similar to the first
screen your app displays. iOS displays this image instantly when the
user starts your app and until the app is fully ready to use.
As soon as your app is ready for use, your app displays its first
screen, replacing the launch placeholder image.
You cannot control the duration, however, if you set a launch image (as you are required to do anyway) and create a special root view controller that displays this same image, then once the launch image is removed your controller is visible. This controller is commonly used in games, perhaps playing some music for a main menu and presenting some options. It will make the launch image transition appear flawless.
