Teste: Adicionando Lightbox ao post_thumbnail

/*
Adicionando lightbox ao post_thumbnail
No functions.php
*/

add_filter( 'post_thumbnail_html', 'cms_post_image_html', 10, 3 );

function cms_post_image_html( $html, $post_id, $post_image_id ) {
    $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id($post_id), 'large');
    $html = '<a rel="lightbox" href="' . $large_image_url[0] . '" title="' . esc_attr( get_post_field( 'post_title', $post_id ) ) . '">' . $html . '</a>';


  return $html;
}


/*
No header.php, adicione jquery, lightbox plugin, seu respectivo css e a chamada para o lightbox, conforme explicado no site do plugin: http://leandrovieira.com/projects/jquery/lightbox/
*/

<script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript" src="js/jquery.lightbox-0.4.js"></script>
<link rel="stylesheet" type="text/css" href="css/jquery.lightbox-0.4.css" media="screen" />

<script type="text/javascript">
jQuery(function($){
    $('a[rel*=lightbox]').lightBox();
});
</script>