What's wrong with the already-existing commercial ones?
Having said that, I must admit that we did study the specs of some of those and found a few gaps here and there (at least so far as our requirement was concerned).
To list a few off the top of my head:
1) Returning of http response codes 2) Following 300 class redirects 3) Control over exact pixel size of the images etc.
We're not really concerned about the existing services adding the missing features - what we really wish to find out is whether or not there exist (at least a decent sized) market for such a service.