Sorry to jump in on this thread; I am also trying to do the same with no luck.
Will be keeping an eye on this thread – hopefully we can solve the problem.
Thanks
I ended up using this code that works great:
$attachment_id = get_sub_field('gallery_image');
$size_thumb = "gallery-img"; // (thumbnail, medium, large, full or custom size)
$size_gallery = "full"; // (thumbnail, medium, large, full or custom size)
$image_thumb = wp_get_attachment_image_src( $attachment_id, $size_thumb );
$image_gallery = wp_get_attachment_image_src( $attachment_id, $size_gallery );
// url = $image[0];
// width = $image[1];
// height = $image[2]; ?>
<div class="gallery-img">
<img alt="Dance Avenue" src="<?php echo $image_thumb[0]; ?>" />
<div class="enlarge-img">
<?php
echo '<a class="fancybox" rel="group-'.get_the_ID().'" href="';
echo $image_gallery[0];
echo '">View Images</a>';
?>
</div>