cuongtran thanks for the suggestion!
We've also got a few complains that current guest avatar selections don't suit for everyone ): So, we need to find a solution. As you know, privacy is the first thing for us when creating any feature. We are concerned about gravatar's privacy issues. There are large databases with md5 hashes, which can be used to find emails from Gravatar hashes (I have read a blogger explain this happen to him, but cannot find the link now).
However, I think we may be able to add Gravatar support as an optional feature. We'll discuss and update this thread when we make a decision 🙂