<?php $banner = get_field('banner'); ?>

<section id="hero">
   <div class="container-fluid">
      <div class="row">
         <div class="col-md-12 principal">

            <img src="<?php echo $banner ?>" width="100%" alt="<?php echo get_bloginfo('name'); ?>">

         </div>
      </div>
   </div>
</section>