something like
$image = wp_get_attachment_image_src(get_sub_field('image', false), 'gallery');
$image2 = wp_get_attachment_image_src(get_sub_field('image', false), 'full');
<a href="<?php echo $image2[0]; ?>" rel="shadowbox[gallery]">
<img class="gallery-images" src="<?php
echo $image[0]; ?>" $alt=<?php
echo get_post_meta(get_sub_field('image', false),
'_wp_attachment_image_alt', true); ?>" /></a>