I don’t know anything about the popup in that theme, I’m assuming that you are supposed to link to a larger size image, so you’ll need to get both sizes. Your code should probably look something like this.
$image = get_field('image');
$small_size = 'cb-480-240'; // (thumbnail, medium, large, full or custom size)
$large_size = 'full';
if ($image) {
$small_image = wp_get_attachment_image($image, $small_size);
$large_image = wp_get_attachment_image_src($image, $large_size);
?>
<a class=”cb-lightbox" href="<?php echo $large_image[0]; ?>
<?php echo $small_image; ?>
</a>
<?php
}