By default images are returned as an array.
You need to either change this to URL, or else you can pull the details from the array to get the URL, eg:
<?php
$image = get_field('ac_imagem_destaque');
if( $image):
$alt = $image['alt'];
$thumb = $image['url'];
$width = $image['width'];
$height = $image['height'];
?>
<img src="<?php echo $thumb; ?>" alt="<?php echo $alt; ?>" width="<?php echo $width; ?>" height="<?php echo $height; ?>" class="thumb_285_350" />
<?php endif; ?>
You can read more here – http://www.advancedcustomfields.com/resources/image/
