I like the idea (a lot), but it would almost double the space that synths use on the storage servers. Currently our photos only make it up to the web once they've been chopped up into deep zoom image tiles. Being that the original photos are jpg and the tiles are also jpg (and jpg uses lossy compression, meaning you can not save it without throwing away some of the information in the file each time you save) the chances are that even if the tiles of the full version of your photo were pieced back together as seamlessly as possible, the result would still not be exactly what you started with.
I say this, not because you wanted to know all of that :), but only to help you understand that you are asking them to suddenly store almost twice the amount of data on their servers. That would certainly eat up your 20 free gigabytes of storage faster.