Markmcl

<?php

add_filter("attachment_fields_to_edit", "add_image_source_url", 10, 2);
function add_image_source_url($form_fields, $post) {
  $form_fields["source_url"] = array(
    "label" => __("Source URL"),
    "input" => "text",
    "value" => get_post_meta($post->ID, "source_url", true),
                "helps" => __("Add the URL where the original image was posted"),
  );