Sounds like a perfect place for microtransactions imo.
You could pay a subscription that gives you access to the microtransactions, that are still paid with real money that you need to convert to an imaginary currency to be used.
So you can pay one ImaginaryCoin to get 1mb more.
[/irony]
Imagine all users that are registered, uploading 5mb each. That is quite a lot, around 8,18TB just for a stupid 5mb picture.