Global Site Search - Replacing Avatar with Featured Image

Hey guys, just a quick one, i've searched the forums for this but the old method is outdated but essentially all i want to do is replace this:-

<a href="<?php echo network_get_permalink() ?>"><?php echo get_avatar( $author_id, 32 ) ?></a>

With just the thumbnail of the object if this makes sense.

Hope someone can point me in the right direction, thank you.

Kind Regards
~Matt