Update postmeta with URL of image attachment

<?php
// http://www.gravityhelp.com/forums/topic/image-path-gform_after_submission#post-79385
// original http://pastie.org/4927121
add_action('gform_after_submission', 'update_form_post', 10, 2);
function update_form_post($entry, $form) {
    if($form['id'] != 20)
        return;

    // update post image
    $postid = $entry['post_id'];
    $postimageurl = wp_get_attachment_url($postid);
    update_post_meta($postid, 'thumb', $postimageurl);
}