Assign Random Avatar or Force Members to Choose One?

[Buck Satan]

I've just added hundreds of new avatars to my vBulletin installation and I'd like to either a) randomly assign one of these avatars to every member who doesn't already use an avatar, or b) force members to pick an avatar if they haven't already chosen/uploaded one previously. My site design puts a lot of emphasis on user avatars and I think it would look a lot more streamlined if everyone had an avatar of some kind. If the a) route if possible, then members can pick or upload a new avatar if they don't like the one randomly assigned to them.

Any help would be much appreciated!