The field you are tying to get is not set for the field user you are trying to show
OR
you are trying to get the author value outside of “the Loop”
<?php
$author_id = get_the_author_meta('ID');
$image= get_field('image', 'user_'. $author_id );
if (!empty($image)) {
?>
<img src="<?php echo $image['url']; ?>" alt="<?php echo $image['alt']; ?>" />
<?php
}