It appears that $field[‘custom_image’] isn’t passing what you want it to. The returned values do not appear to be correct.
You want to put the key of the meta value you need as the second variable in the get_post_meta() function, for example:
$image_upload = get_post_meta($post->ID, '_my_image_id', true);
Each custom meta field you create has a unique identifier, thats the value you want to pass in.
You should also check to see if the returned value of $image_upload is false, in which case the meta was not set for that particular post.