With Retina.js in the mix, retina images are picked up automagically.

picturefill-better.php

<?php 
    $images = get_children(array(
        'post_parent' => get_the_ID(),
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
    );
    if ( $images ) : ?>

    <?php foreach( $images as $image ) :  ?>
        <span data-picture>
            <span data-src="<?php echo wp_get_attachment_image_src($image->ID, 'small'); ?>"></span>
            <span data-src="<?php echo wp_get_attachment_image_src($image->ID, 'medium'); ?>" data-media="(min-width: 769px)"></span>
            <span data-src="<?php echo wp_get_attachment_image_src($image->ID, 'large'); ?>" data-media="(min-width: 991px)"></span>

            <noscript>
                <img src="<?php echo wp_get_attachment_image_src($image->ID, 'medium'); ?>"/>
            </noscript>
        </span>
        
    <?php endforeach; ?>
<php endif; ?>