<img class='attachment-image' src='<?php echo wp_get_attachment_url(get_post_thumbnail_id(${post_ID})) ?>'/>
atau
<?php the_post_thumbnail(); ?>
See more info the_post_thumbnail function : http://codex.wordpress.org/Function_Reference/the_post_thumbnail
Get image attachment url :
//$var = wp_get_attachment_image_src( get_post_thumbnail_id(), '${image_mode}');
//${image_mode} : small, medium, large, thumbnail
//
//example :
$large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large');
echo $large_image_url[0]