After receiving the url, you can replace http with https preg_replace()with PHP.
$cover = $fql_query_obj[cover][source];
$secure_cover = preg_replace('/^http(?=:\/\/)/i','https',$cover);
As far as I tested, it works great.
javascript.
cover = cover.replace(/^http:\/\//i, 'https://');
, -:)