How can i link a custom post type thumbnail to intermediate size

Question

I'm trying to build a picture gallery with custom post type in my theme and I think i figure it out almost everything but the call in the template, I already have the custom post type in the functions.php, I have a Page with a custom call to the post type and it’s working (the post are showing in the page with the template) what I cannot figure it out it’s the call for the href that should go to open the image bigger probably with some modal like lightbox or something like that.

I have to mention that I’m trying to use feature image and already have created some image sizes for it in the functions.php, I’m going to paste the code for you to understand… Thank you in advance

Custom post type template

<?php
/*      
    Template Name: test Post Type Gallery   
*/
get_header(); 
?>
<div id="container">
    <div id="content" role="main">

        <?php $loop = new WP_Query( array( 'post_type' => 'bird', 'posts_per_page' => 10 ) ); ?>
        <?php while ( $loop->have_posts() ) : $loop->the_post(); ?>

        <ul class="gallery-ul">
            <li class="the-picture"><?php the_title( '<h2 class="picture-title"><a href="' . get_permalink() . '" title="' . the_title_attribute( 'echo=0' ) . '" rel="bookmark">', '</a></h2>' ); ?>
            <a href="-----?????????--------"> <?php the_post_thumbnail(array(200,200)); ?></a></li>
        </ul>

        <?php endwhile; ?>

    </div><!-- #content -->
</div><!-- #container -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>

Functions.php
This is for the Size of the Images

if ( function_exists( 'add_theme_support' ) ) { 
    add_theme_support( 'post-thumbnails' );
    set_post_thumbnail_size( 250, 250, true ); // Normal post thumbnails
    add_image_size( 'medium-picture-thumbnail', 400, 400 ); // Medium thumbnail size
    add_image_size( 'big-picture-thumbnail', 700, 700 ); // Big thumbnail size
}

Answer 1

                    <a rel="lightbox" href="<?php $image_id = get_post_thumbnail_id();
                $image_url = wp_get_attachment_image_src($image_id,'large', true);
                echo $image_url[0];  ?>">
                <?php the_post_thumbnail('thumbnail'); ?>
                </a>

Yeah the above (posted by @null) is correct on getting the url, here it is all put together. Including the rel=lightbox. I call to WP's large image size here personally for the lightbox image to be linked to.
the_post_thumbnail wp_get_attachment_image_src

Answer 2

So by the sounds of it your having problems with the function the_post_thumbnail. Essentially you just need that to return a URL rather than the image. Give this a try:

$src = wp_get_attachment_image_src( get_post_thumbnail_id($loop->the_ID()), array( 800,600 ), false, '' );
echo $src[0];

Set the array(width,height) to your desired image size and echo $src[0] into your href="--------???????---------" attribute.