It looks interesting but I found the documentation page to be pretty lacking. The simple example they show just allows you to pass a height and width parameter, but the home page says you can do cropping, watermarking, etc.
You can pass other info in the outputs hashes, like labels, your own ids, or whatever. That extra info will be passed back to you in the response JSON.
So I assume it is a case of emailing them and asking for an exhaustive list of what can be passed to their api, the documentation definitely needs work.