What did you get if you
<?php
echo "<pre>";
print_r($hero);
echo "</pre>";
?>
BTW: There’s one index to much echoing the $hero parameters:
<?php
// vars
$hero = get_field('hero');
if( $hero ): ?>
<div id="hero">
<img src="<?php echo $hero['url']; ?>" alt="<?php echo $hero['alt']; ?>" />
</div>
<?php endif; ?>
