This will do it:
<?php
while(has_sub_field('diaporama')):
$image = get_sub_field('photos');
$image_full = $image['sizes']['large'];
$alt = $image['title']; ?>
<div class="col-split">
<a class="fancybox" rel="gallery1" href="<?php echo $image_full; ?>">
<img class="img_cen" src="<?php echo $image_thumb; ?>" alt="<?php echo $alt; ?>" title="<?php echo $alt; ?>" />
</a>
</div>
<?php endwhile; ?>
<?php endif; ?>
