iPhone HTML apps startup image FAQ: How do I set the startup image (splash screen) for my iPad or iPhone HTML app?
This quick blurb from the Apple developer documentation leads us to the answer:
On iOS, similar to native applications, you can specify a startup image that is displayed while your web application launches. This is especially useful when your web application is offline.
By default, a screenshot of the web application the last time it was launched is used. To set another startup image, add a "link" element to the webpage.