Hi,
If you are willing to go with Meteor instead of Express I can get this done ~budget, in two weeks. Meteor is rapid to build with, highly functional and scalable, and an all around excellent platform/framework.
I just joined Freelancer, but I've been a developer for the last ten years.
For the MVP if we're going with Meteor I'd recommend images in GridFS to start, and AWS to grow. Throw the first round on Modulus and let them handle scaling while you see where this is going to go.
If you have any questions just let me know.
Topher