In the most recent iPhone and iPod Touch software upgrade, Apple added the ability to show a startup image for websites saved to a user’s home screen. Until now, these full-screen web apps would simply show a (poorly) compressed screenshot of the previous page viewed or nothing at all. To add a startup screen, simply include a the following tag in the head of your document and create an image in the location referenced by href:
<link rel="apple-touch-startup-image" href="startup.png" />
The PNG can be named anything, but it must be in PNG format and it must be 320×460 pixels. Technically, it seems the PNG displays until the document triggers the ‘ready’ callback, before the window’s ‘loaded’ callback.