<?php
$images = get_field('gallery');
if( $images ): ?>
<ul>
<?php foreach( $images as $image ): ?>
<li>
<a href="<?php echo $image['url']; ?>">
<img src="<?php echo $image['sizes']['thumbnail']; ?>" alt="<?php echo $image['alt']; ?>" />
</a>
<p><?php echo $image['caption']; ?></p>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?><?php
$images = get_field('gallery');
$size = 'full'; // (thumbnail, medium, large, full or custom size)
if( $images ): ?>
<ul>
<?php foreach( $images as $image_id ): ?>
<li>
<?php echo wp_get_attachment_image( $image_id, $size ); ?>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>