You need to create an image (use a custom control and render an image from it) and use it has a bakcground.
This link could help for generating the image: http://spasol.wordpress.com/2013/06/24/creating-custom-live-tiles-for-windows-phone/
And here are the template available for universal app : http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh761491.aspx