AuthorUserpicAsset
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>