<div id="online">
<?php
global $bp;
function check_is_user_online($user_id){
if ( bp_has_members( 'type=online&include='.$user_id) ) return true;
else return false;
}
$this_id = bp_displayed_user_id();
$is_online = check_is_user_online($this_id);
if ($is_online) echo "<img src='http://www.yourweb.com/wp-content/themes/yourtheme/_inc/images/online.png' />";
else{ echo "<img src='http://www.yourweb.com/wp-content/themes/yourtheme/_inc/images/offline.png' />";}
?>
</div>