iPhone Development 101

iPhone Development 101: User Interface:
Default (Launch) Image Sizes for iPhone & iPad

The launch image (or default image) is displayed while your app launches. (If no default image is provided, then a black screen is displayed.)
To include a launch image in your app, add images of the appropriate size for the device, and name them as follows:
Screen sizes (in pixels) of the iPhone 4S and iPhone 5

iPhone 4S
3.5" display
iPhone 5
4" display
Non-retina 320 x 480 pixels
Retina 640 x 960 pixels
640 x 1136 pixels
Note that the Default-568h@2x.png image is required for an app to display in full-screen mode on the iPhone 5.
Launch images for the iPhone are only accepted in portrait mode. If you want to provide a landscape launch image, create the image and rotate it 90 degrees in an image editor, then save as named above.
Screen sizes (in pixels) iPad

iPad - Portrait iPad - Landscape
Non-retina 768 x 1004 pixels
1024 x 748 pixels
Retina 1536 x 2008 pixels
2048 x 1496 pixels
iPad launch images do not include the status bar area. You can provide different launch images for each orientation (Portrait, PortraitUpsideDown, LandscapeLeft, and LandscapeRight); consult the Apple documentation for details.

Additional References