An example on how to register your own attachment as feature image from the Wordpress media lightbox.

admin.js

// Pick up data from service.php and send returned data to wp ajax admin.

var win = window.dialogArguments || opener || parent || top

$.ajax({
    url: ajaxurl,
    type: "POST",
    contentType: "application/x-www-form-urlencoded; charset=UTF-8",
    data: postData,

    success: function(str){
        if ( str === '0' ) {
            alert(setPostThumbnailL10n.error);
        } else {
            jQuery('a.wp-post-thumbnail').show();

            win.WPSetThumbnailID(postId);
            win.WPSetThumbnailHTML(str);
            win.tb_remove();
        }
    }
});

service.php

<?php

// Return this data to js, so we can perform a wp ajax request.
// Can be used in conjunktion with my save image to wordpress gist (https://gist.github.com/marteinn/6174127).

$data = array(
    "post_id" => $post_id,
    "thumbnail_id" => $attachment_id,
    "_wpnonce" => wp_create_nonce("set_post_thumbnail-".$post_id),
    "action" => "set-post-thumbnail"
);

echo json_encode($data);