<?php
// ALL parameters are optional. Not needed if you are on WP LOOP
echo get_the_post_thumbnail_url( $the_query->ID, array( 500, 400) ); ?>
<?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; ?>
$img_url = wp_get_attachment_image_url(get_post_thumbnail_id(get_the_ID()), 'full');
<img src="<?PHP echo $img_url?> ">
//Default WordPress
the_post_thumbnail( 'thumbnail' ); // Thumbnail (150 x 150 hard cropped)
the_post_thumbnail( 'medium' ); // Medium resolution (300 x 300 max height 300px)
the_post_thumbnail( 'medium_large' ); // Medium Large (added in WP 4.4) resolution (768 x 0 infinite height)
the_post_thumbnail( 'large' ); // Large resolution (1024 x 1024 max height 1024px)
the_post_thumbnail( 'full' ); // Full resolution (original size uploaded)
//With WooCommerce
the_post_thumbnail( 'shop_thumbnail' ); // Shop thumbnail (180 x 180 hard cropped)
the_post_thumbnail( 'shop_catalog' ); // Shop catalog (300 x 300 hard cropped)
the_post_thumbnail( 'shop_single' ); // Shop single (600 x 600 hard cropped)
//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']), '');
}