Absolute path for timthumb script in sitewide tags blog

// transforms a relative image path into an absolute one for running timthumb on a parent WP multisite blog, which has been populated by the sitewide tags plugin

function dyn_getimagepath($img_src) {
    if ($post_id == null) {
        global $post;
        $post_id = $post->ID;
    }
    global $post_blog_id;
    global $wpdb;

   $post_blog_id = get_post_meta($post->ID, 'blogid', true) ;

    if (isset($post_blog_id) && $post_blog_id > 0) {
        $imageParts = explode('/files/', $img_src);
        if (isset($imageParts[1])) {
            $img_src = '/wp-content/blogs.dir/' . $post_blog_id . '/files/' . $imageParts[1];
        }
    }
    return $img_src;
}