Building the post thumbnail markup based on the original size

functions.php

<?php
/**
 * Given a post ID, return the permalink for the original size post thumbnail
 *
 * @param int $ID post id
 * @return string deep link for a post thumbnail
 */
function _thumb_link($ID) {
    $toReturn = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
    return $toReturn[0];
}

post.php

<?php
$ID = get_the_ID();

$postLink = get_permalink();

$thumb = !empty(_thumb_link($ID))
    ? _thumb_link($ID)
    : 'http://placehold.it/255x130';
?>

<a href="<?php echo $postLink; ?>" class="entry--thumb">
    <img
        class="entry--thumb_img"
        src="<?php echo $thumb; ?>"
    />
</a>