You could just use FQL, it is my preferred method and offers a lot of flexibility. This code sample should be what you need:
$album_id = '6464156415616';
try{
$fql = 'SELECT pid, src_small FROM photo WHERE pid in (SELECT cover_pid FROM album WHERE aid='.$album_id.' AND owner=me())';
$param = array(
'method' => 'fql.query',
'query' => $fql,
'callback' => ''
//,'return_ssl_resources'=>1 //set this option if you want the source to come from https
);
$fqlResult = $facebook->api($param);
$fqlResult['type'] = $uploadVal['type'];
//fetch values
$src_small = $fqlResult['src_small'];
} catch(Exception $o){
print_r($o);
}
You can find a reference to all of the FQL tables here: http://developers.facebook.com/docs/reference/fql/