No. You probably wouldn't want that. Noting wrong with giving people a bit of time before deciding which avatar is right for them.
Maybe you could setup a cron job where 3 weeks after registering if you have over 20 posts but haven't uploaded an avatar .... a random ugly avatar is inserted.
- a picture of britney spears with her head shaved.
- random pictures of Wayne or Garth look alikes
People will detest their image enough to change it !