Indeed, the storage and retrieval of similar images is the hardest part. I do not know of a single networked open-source storage solution for this. I really wish that there was a project with a mindset of Redis, but for MVP trees. By the way, may it be possible to implement MVP data structure in Redis, as the project is now? I can not think of possible replication issues with this, apart from the fact that one would have to pre-define a metric space for every tree.

It could be a great extension to Redis DSL.