Remote site snapshot using Wordpress API shortcode

functions.php

function wpr_snap($atts, $content = null) {
        extract(shortcode_atts(array(
            "snap" => 'http://s.wordpress.com/mshots/v1/',
            "url" => 'http://www.sagive.co.il',
            "alt" => 'My image',
            "w" => '400', // width
            "h" => '300' // height
        ), $atts));

    $img = '<img src="' . $snap . '' . urlencode($url) . '?w=' . $w . '&h=' . $h . '" alt="' . $alt . '"/>';
        return $img;
}

add_shortcode("snap", "wpr_snap");
//how to use: [snap url="http://www.example.com" alt="Cool Site!" w="300px" h="200px"]