Edita memberlist.php
busca:
$userlist .= "<td valign=\"top\" align=\"center\">".$user_name."</td>\n";
y lo reemplazas por:
$user_avatar = (!$user_row['user_avatar'] || $user_row['user_avatar'] == "blank.gif") ? "" : "<img src=\"".TEMPLATE_PATH."/avatars/".$user_row['user_avatar']."\">";
$userlist .= "<td valign=\"top\" align=\"center\">".$user_name."<br/ >".$user_avatar."</td>\n";