Prueba así:
Código PHP:
$name = array_pop(explode('/', urldecode($url)));
$img = urldecode($url);
$size = filesize($img);
header("Pragma: ");
header("Cache-Control:");
header("Content-length: $size");
header("Content-type: application/force-download");
header('Content-Disposition: attachment; filename="'.$name.'"');
header("Content-Transfer-Encoding: binary");
$fp=fopen("$img", "r");
fpassthru($fp);
fclose($fp);