This looks pretty cool. I'm not a designer, so I don't know what's on the market already. But I have to say this is a simple (but real) problem for which you've provided an interesting solution.
I haven't quite found a hosted solution to this particular problem. There are many web apps, and desktop apps, that allow you to add comments and annotations to images and do project management that way - but I haven't found one that combines that with the management of multiple versions.
I am willing to be proven wrong though, so if anyone has any counter examples, please pass them along :)