Make sure your image is 480x320 pixels, and placed in your application at the top level. The frameworks will display this image until your initial view has loaded and is ready for display.
If you are seeing a black background then there is something amiss, however if you are just seeing your app's initial view then be happy you have managed to launch your app and have it ready for use in good time.
If you really, really want the user to to see your splash screen then slow down your launch by adding a delay into your app before you present the initial view
The file name must be Default.png (with a capital D). Also, do a clean/rebuild of the entire project to ensure the most recent version of the file is being included in the project. Check the filename on the filesystem, it may display differently in XCode.