I’ve written a tiny web application called ‘Avatar Harmony’.
You can sign in with twitter or facebook, add your Gravatar login details, and the app will automatically sync your profile pics across each of your accounts.
It can sync avatars in the following directions:
Unfortunately, the Facebook Graph API doesn't let us update profile pics at the moment, but I'll keep looking around for a solution.
Try it out at https://avatarharmony.com!