Custom Field – Thumbnails

Hi

currently code i use for my custom field (thumbnail) is

<?php if(get_post_meta($post->ID, "thumbnail", true)) {
$size = getimagesize($_SERVER[""].get_post_meta($post->ID, "thumbnail", true)); ?>
<img src="<?php echo get_post_meta($post->ID, "thumbnail", true); ?>" alt="<?php the_title(); ?>" width="<?php echo $size[0]; ?>" height="<?php echo $size[1]; ?>" />
<?php } ?>

Question here. How to make it will appear No Thumbnail Image if we not fill the input on custom field?