This container tag creates a context that contains the userpic asset for the current author. This then allows you to use all of Movable Type's asset template tags to display the userpic's properties.

    <ul><mt:Authors>
         <mt:AuthorUserpicAsset>
           <li>
             <img src="<$mt:AssetThumbnailURL width="20" height="20"$>"
                width="20" height="20"  />
             <$mt:AuthorName$>
           </li>
         </mt:AuthorUserpicAsset>
    </mt:Authors></ul>