The screenshot thing is very disconcerting in HTML webapps: when you click on the app to go back to it, you are shown the screenshot and think you can interact with it, but the webview and app are then (visibly) cleared and reloaded. It looks and feels horrible. I'm not sure if there are ways to improve this with their meta tags.
You can specify apple-touch-startup-images that load on web app startup. These ideally consist of your app's chrome with any dynamic or textual elements removed.
