I was just having a think about your white theme and it occurred to me that it might be an idea to shift all of the default images into a folder called "default" (next to the folder called "white).
You could then provide a selector within the app along these lines:
Default theme for all new users [default]
Allow Users to change theme? [Y/N]
This would allow really easy theme selection from within the site rather than changing the config.
It would also meant the path to your file would simply be "icon.png" and HAP would know the rest.
It would also allow people to share their themed app folders by zipping and unzipping.
Finally, end-user theme selection would allow for example a "High Contrast" theme to be crafted.
Not a particularly feature rich suggestion, and i understand it would be quite a bit of work..
Ooh, i missed the URL on my phone - more stuff to play with! :)