Mockup Generation service (API First)

I am trying to create something like mediamodifier.com. Please study API documentation of the same and discuss software architecture of the same. This should be done using Photoshop Automation in my opinion. Users should be able to provide PSD files (with whatever layer specifications that the system needs) to generate renders.

Apply only if you have done something similar. It should be able to balance the load between multiple machines, if / when required.