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>