Resizing startup screens is very boring. This is a set of actions that will copy and resize your portrait and landscape startup screens to the various dimensions that you need for tablet (non-retina) and mobile (retina and non-retina).
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Copy the .atn file somewhere useful and load the actions into Photoshop via Actions panel > Load Actions...
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Create your start screen in the largest sized portrait (1536 x 2008).
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Also create your largest sized landscape (1496 x 2048, content should be rotated 90 degrees clockwise so the left edge is at the top). The proportions are thinner and taller so you'll need to tweak the background manually.
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Go back to the portrait version and run the "apple start images - portrait" action. It creates 3 new documents.
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Check the mobile versions (apple-touch-startup-retina and apple-touch-startup) as the width gets clipped a little.
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Go back to the landscape version and run the "apple start images - landscape" action. It creates 1 new document.
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Save for Web and Devices in the png format for all images.
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(Devs) Use some JavaScript along the lines of h5bp/mobile-boilerplate#94 in the head of the HTML to output just 1 startup image. A rare legitimate use of document.write. If you list them all, they all get downloaded to the device.