An Idea for Open Avatar Exchange

I am using GRAVATAR at github.com.
It is very interesting service that provides globally recognized avatar image from a hash of email address.
What is clever is a service using GRAVATAR can present its user's avatar without not exposing his/her email address.

But the GRAVATAR is not the only provider of such avatars.
For example,
twitter.com has huge database of mapping from email address to avatar image.
There are a lot of services which have such the database.

Imagine twitter.com, facebook, linkedin and so on provide their avatar images by the same manner.
We can provide avatar image for every user of which we know his/her email address everywhere and quite easily.
This is the principal advantage of this idea.

But what is advantage for provider?
That's the obvious question.
Answer is inter-service completion of avatar images.
In each service, not all users have his/her avatar image.
For such users, the service provides disappointing default avatar image.
But if there is the open avatar exchange, the service can try to find it in other services instead.

The way to make the exchange is really simple.
Provide REST API to map MD5 hash of email address to avatar image.
Block accesses from domains you don't want to exchange.

In addition, avatar providers can harvest stats of the sites that are using your avatar image.