1 - Create the shortcode. Paste the code in your functions.php file

function wpr_snap($atts, $content = null) {
    extract(shortcode_atts(array(
                "snap" => 'http://s.wordpress.com/mshots/v1/',
                "url" => 'http://www.sharemycode.com',
                "alt" => 'My image',
                "w" => '400', // ancho
                "h" => '300' // alto
                    ), $atts));
    $img = '<img src="' . $snap . '' . urlencode($url) . '?w=' . $w . '&h=' . $h . '" alt="' . $alt . '"/>';
    return $img;
}
add_shortcode("snap", "wpr_snap");

2 - You can use the snap shortcode in your post or page

[snap url="http://www.sharemycode.com" alt="Descripción" w="400" h="300"]