List of authors in a category

I'm listing recent authors per category on my site, but it needs some tweaks. I grabbed code from the MT wiki for this: http://wiki.movabletype.org/Create_a_List_of_Authors_in_a_Category and adapted it such that the list of author icons only includes those authors that have uploaded an avatar. I'd like to limit the number displayed to 12, without duplicating icons. Not sure if that's possible, since unique="1" won't work on the entries tag.

Example: http://beta.davidsuzuki.org/share/

<mt:Authors role="!(Blog Administrator)" sort_by="created_on" sort_order="descend">
  <mt:SetVarBlock name="new_author"><mt:AuthorName /></mt:SetVarBlock>
  <mt:Entries author="$new_author" lastn="1">
      <mt:If tag="AuthorUserpicURL"><li><a href="<$mt:Var name="profile_view_url" encode_html="1"$><$mt:AuthorID$>" class="name" title="<$mt:AuthorDisplayName$>"><$mt:Include module="Userpic" userpic_size="50"$></a></li></mt:If>
  </mt:Entries>
</mt:Authors>