ACF Plugin – Random Gallery Image with wp_get_attachment_image()

<?php 
    $images = get_field('gallery');
    $size="full"; // (thumbnail, medium, large, full or custom size)
    $rand = array_rand($images, 1);

    if( $images ): ?>
            <?php echo wp_get_attachment_image( $images[$rand]['ID'], $size ); ?>
<?php endif; ?>

This code should work. array_rand() return key if second param set to 1 or array with keys if second param > 1