Código PHP:
<?
$tam=getimagesize("http://img163.imageshack.us/img163/2126/adictomusicalqi0.gif");
$ancho = $tam[0];
$alto = $tam[1];
echo "<script>window.open(\"http://img163.imageshack.us/img163/2126/adictomusicalqi0.gif\",\"nombre\",\"width=$ancho, height=$alto\")</script>";
?>
ya lo probe y funciona, tal vez tu explorador blockea el popup