placing the media uploads in a subdirectory

Add the following to your theme’s functions.php file, making sure you replace the example CDN URL with your own:

function my_cdn_upload_url() {
    return 'http://mk124.yourcdn.com/yoursite/wp-content/uploads';
}
add_filter( 'pre_option_upload_url_path', 'my_cdn_upload_url' );