You can start by trying this.
1. Change this <span class="id_53">Display Post Image</span>
to <span class="id_53" data-id="53">Display Post Image</span>
2. Create you js file
$(function () {
// click event example
$('.id_53').on('click', getThumb);
// call the ajax
function getThumb() {
$.post( 'http://path/wp-content/themes/your-theme/ajax-file.php' , 'my_id='+$(this).attr('data-id') , showThumb );
}
// get image source
function showThumb(e) {
$('#img_in').style({'background-image':e});
}
});
3. Create your ajax-file.php
// for external files include( '../../../wp-load.php' ); // get your variable $my_id = intval( $_POST['my_id'] ); // get thumbnail ID $thumb_id = get_post_thumbnail_id( $my_id ); // get path of thumbnail $img = wp_get_attachment_image_src( $thumb_id, 'full' ); echo $img[0];