KORREKTUR:
$image_size = getimagesize($values['file']['filename']);
$image_width = $image_size[0];
$image_height = $image_size[1];
<img src="<?php echo $values['file']['filename']; ?>" alt="performance_muenster_2016" width="<?php echo $image_width?>" height="<?php echo $image_height?>"/>