Nonfunctional WordPress get_image_tag filter

/**
 * Filter to get caption and append image information to it.
 *
 * @returns text HTML content describing with media attribution
 **/

add_filter('get_image_tag', 'append_media_credit', 10, 2);

function append_media_credit($html, $id) {

    $media_string = media_credit_string( $id );

    $html = $html . $media_string;

    return $html;

}

/**
 * Function to lookup media creator fields and put them together.
 *
 * @returns an html string with creator name and license name links.
 **/

function media_credit_string( $media_id ) {

    $name = get_post_meta( $media_id, 'creator_name', true );
    $name_url = get_post_meta( $media_id, 'creator_url', true );
    $license_name = get_post_meta( $media_id, 'license_name', true );
    $license_url = get_post_meta( $media_id, 'license_url', true );

    $creator = null;
    if( $name ) {
        if( $name_url ) {
            $creator = ' <a href="' . $name_url . '">' . $name . '</a>.';
        } else {
            $creator = ' ' . $name;
        }
    }

    $license = null;
    if( $license_name ) {
        if( $license_url ) {
            $license = ' <a href="' . $license_url . '">' . $license_name . '</a>.';
        } else {
            $license = ' ' . $license_name;
        }
    }

    return '<span class="mrw-media-attr">Credit:' . $creator . $license . '</span>';

}