It's a really clever solution, kudos to them. Is there a limitation why they couldn't just have the notification point to a remote URL and update server-side using whatever solution they wanted? It wouldn't have been as elegant but may have been easier to build.
The main reason is that the phone will download the image each time you post a notification, and with 650 alerts throughout the night (for 650 constituencies) the user would end up downloading a lot of data with very little to show for it (since newer notifications replace the older ones).
But to add to that, it needed to be customised for different screen sizes. So we'd have to have generated a lot of versions server side.
