<?php
// change 'image_size' to your custom size
// change 'holding.jpg' to your holding image

$thumb_id = get_post_thumbnail_id();
$thumb_url = wp_get_attachment_image_src($thumb_id,'image_size', true); 
?>

<img src="<?php bloginfo('stylesheet_directory'); ?>/holding.jpg" data-lazy="<?php echo $thumb_url[0];?>"/>