Yes, sorry, perhaps I should have been more clear. I’m not sure how to make the $rand variable apply. I tried things like this, to no avail:
<?php
$images = get_field('gallery', 'option');
$size = 'full'; // (thumbnail, medium, large, full or custom size)
$rand = array_rand($images, 1);
if( $images ): ?>
<?php echo wp_get_attachment_image( $images[$rand], $size ); ?>
<?php endif; ?>
