Repeater field with image size
<?php echo " <ul class=slider>"; ?>
<?php
$post = $wp_query->post;
$rows = get_field('tombstones', $post->ID);
if($rows) $i = 0; { shuffle( $rows );
foreach($rows as $row) { $i++; if($i==16) break;
$image_big = wp_get_attachment_image_src( $row['tombstones_image'], 'full' );
$image = wp_get_attachment_image_src( $row['tombstones_image'], 'tombstone_thn' ); ?>
<li class="attachment">
<a href='<?php echo $image_big[0]; ?>'>
<img class="colorbox_link" src="<?php echo $image[0]; ?>" />
</a>
</li>
<?php } } ?>
<?php echo "</ul>"; ?>