Apple's iOS Human Interface Guidelines states the dimensions for launch image for each screen dimension. While iPads have 2 launch image for portrait and landscape, iPhones and iPod touch only have 1 for each screen size. Specifically for pre-iPhone 5 retina devices, only create 640x960 and 320x480. You should not create a launch image with the dimensions 640x920 or 320x460. If you do, and the user launch the app with the in-call (or tether) indicator on, you will notice that the middle of the launch image will be squashed. It's especially obvious if you have a logo or words at the center of your launch image.

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