It would be neat to consider other types of images you could "clean up" or alter for people - sort of like a Posterous-meets-Photoshop app. An obvious example that comes to mind would be to remove red eye from pictures. I know a lot of photo editing apps can do this already, but just think how much easier it would be for most people to just send an email and get a cleaned up version back. Boom, done.
Don't know how feasible this is, and you've got a lot of room to grow in your current segment already, but would be neat to see where else this could be applied to!
Thanks for the suggestions. I had thought of a couple of things: simple 90/-90 rotations, removing red eye was one of them - I have to figure out an automated way of doing that. Thanks for checking it out!