Use a splash screen image with the file name SplashScreen.png with the dimensions 620x300px.

Ensure the splash screen image is a transparent image because the image will not cover the entire screen. The operating system fills the background and draws the splash screen image centered on the device screen. By default, the background color is set to black, but you can set a different color inside the Visual Studio project.

For more information, refer to information about splash screens on the Microsoft website at http://msdn.microsoft.com/en-us/library/windows/apps/br211467.aspx and http://msdn.microsoft.com/en-us/library/windows/apps/hh465338.aspx.