If only the author would implement the proper google app engine image caching (via the get_serving_url) method. This is a true cdn (in that content is held close to the user) and allows dynamic image sizing by changing the URL!
Right now there are a few other conditions that need to be fixed (the data store being full, adding a purge-least-used, handling the fetch API failing, etc, etc) before I make any changes to how the data is stored.
I know from experience that image delivery times are significantly reduced using the get_serving_url method, so it could be a killer feature.
The hard bit will be that the cdn distributed URLs are not guessable, so you would need to communicate the cdn URL back to the app.