The name of the taxonomy for tags is actually “post_tag” so getting the field could look something like this
$image = get_field('tag_img', 'post_tag_'.$tag->term_id);
however, a lot has changed in 2 years. You could also use
$image = get_field('tag_img', 'term_'.$tag->term_id);
or even
$image = get_field('tag_img', $tag);