Wordpress: Mosrar thumbnails y al hacer click salga grande

if(has_post_thumbnail($post_array[$i]->ID)):
    $thumbnail = wp_get_attachment_image_src(get_post_thumbnail_id($post_array[$i]->ID), 'thumbnail');
    $fotoFinal = wp_get_attachment_image_src(get_post_thumbnail_id($post_array[$i]->ID), 'large');
    $urlTN = $thumbnail[0];
    $urlFF = $fotoFinal[0];
endif;

echo "<p><a href='".$urlFF."'><img src='".$urlTN."'></a></p>";