if ( has_post_thumbnail()) {
$large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large');
echo '<a href="' . $large_image_url[0] . '">';
the_post_thumbnail('thumbnail');
echo '</a>';
}
<?php
echo get_the_post_thumbnail( $post_id, 'thumbnail' );
?><?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
<div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">
</div>
<?php endif; ?><?php echo the_post_thumbnail(); ?>get_post_thumbnail_id($post) //default
$image_url = wp_get_attachment_url(1234);
// change if post have feature image
if (has_post_thumbnail($recent_post['ID'])) {
$image_url = wp_get_attachment_url(get_post_thumbnail_id($recent_post['ID']), '');
}