I tried the above change but the url is not been shown at all – the title works?
<?php
$back = get_field('bg_image');
$size = 'full';
if( $back ) { ?>
<div class="bg-image" style="background-image: url('<?php echo $back['sizes']['full'];?>');">
<div class="center-wrapper">
<h1><?php the_title(); ?></h1>
</div>
</div>
<?php } ?>