Gallery images loop problem

I am looping through a custom post gallery on my single portfolio page.
Like this:

<?php $gallery = get_post_gallery_images();
        foreach($gallery as $img){ ?>
            <div class="galerija-box">
                <a href="<?php echo $img ?>" rel="lightbox"><img src="<?php echo $img ?>"/></a>
            </div>
        <?php } ?>

so i need a way to have 2 different $img urls, the a href to be the fullsize and the img src to be thumbnail?

Any help?