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 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) <?php if(has_post_thumbnail()){
the_post_thumnbail('cstm_name', array( 'class' => 'class1 class2' ))
} else { ?>
<img class="card-img-top" src="http://placehold.it/750x300" alt="Card image cap" class="img-responsive">
<?php } ?>