There really is no way because they can be using an offsite one too. Just leave it and if you run into a duplicate or feel like changing it you can change it later.
Hey, Craven! My avatar's gone again and this time it's not on the Christmas avatar thread either. What happened to my Christmassy penguin????? Lots of other people's are gone, too.
Jespah, Walter and all other kind welcomers: I have to confess--I USED to have red hair...and this is not what I looked like then--this is a gif I found. Plus, I am not sure if my tongue is that long!! LOL!
Note: I just added code so that if the remote avatar is missing it will not display the red x but simply no avatar at all. This will make things load faster and look better if the site where your avatar is hosted goes down.