You can do this in 7.3 by creating a new group and allowing users to upload avatars after they reach your post target of 100 (or whatever limit you decide).
In Group Management create a new group, put 100 in the Post Count field, copy all your settings from your regular group (e.g. Users) but in the [Site] permissions change Can specify a remote avatar or Can upload an avatar from 0 to 1.
