Please try the following code.
<?php
$thumbnail_id = get_post_thumbnail_id(get_the_ID());
if (!empty($thumbnail_id))
{
$thumbnail = wp_get_attachment_image_src($thumbnail_id, 'full');
if (count ($thumbnail) >= 3)
{
$thumbnail_url = $thumbnail[0];
$thumbnail_width = $thumbnail[1];
$thumbnail_height = $thumbnail[2];
$thumbnail_w = 620;
$thumbnail_h = floor($thumbnail_height * $thumbnail_w / $thumbnail_width);
}
}
if (!empty ($thumbnail_url)): ?>
<img class="thumbnail" src="<?php echo $thumbnail_url; ?>" alt="<?php the_title_attribute(); ?>"
width="<?php echo $thumbnail_w; ?>" height="<?php echo $thumbnail_h; ?>" />
<?php endif; ?>
http://www.boxoft.net/2011/10/display-the-wordpress-featured-image-without-stretching-it/