I wonder why are there so many services that work with this kind of business model. while I would be willing to pay for code I can own, I would never invest in something in something where I can't predict it's future.
There are at least couple open source projects that handle dynamic image generation on the backend quite nicely. It seems photon is dead, but I found it works quite well and is easy to extend. Thumbor seems quite active although I've never used it myself.