1. Registered Usergroup cannot use Avatars
2. Usergroup "Normal with Avatars" can
3. Create a Promotion to promote Users from Registered to "Normal with Avatars" after X posts
In Templates postbit/postbit_legacy/MEMBERINFO/memberlist_resultsbit add
Code:
<if condition="$arrayname['usergroupid'] == 2">
Default Avatar Code here
</if>
$arrayname is $post or $userinfo, depending on the template.
Done.