I'm a PHP developer working for John. I've just started a modification of the marketpress plugin that would allow the option for image uploads on a per product basis. A product could be something like a custom greeting card or a journal cover that a customer can put their own image on.
I've been told that this is a feature currently in development for a future version, and I would love to contribute to the project if there's a beta version of this functionality in the works.
If Aaron has some free time, I'd love to mutually help us both out!