Guayo,
Sorry for the delay in getting back to this. The code below should work for fetching an avatar from the social network for a user. You'll need to test the cases where a user hasn't connected with Facebook or Google and ensure it works how you want. I tested this quickly, but definitely not enough:
$fbId = JFBCFactory::usermap()->getProviderUserId($jUserId, 'facebook');
if ($fbId)
$avatarUrl = JFBCFactory::provider('facebook')->profile->getAvatarUrl($fbId, true);
The $avatarUrl will either be null (empty) or the actual URL of the user's avatar.
In the above, you can change 'facebook' to 'google' to do the same for Google.
Let me know how that goes or if you run into any other issues.
Thanks,
Alex