Image field should return image url, not id or array.
If you want more control you can set to return id and from that
<?php
$imgID = get_sub_field('background_image');
$imgSize = "full"; // (thumbnail, medium, large, full or custom size)
$imgArr = wp_get_attachment_image_src( $attachid, $size );
// url = $image[0];
// width = $image[1];
// height = $image[2];
?>
<?php
if( !empty($image) ): ?>
<div id="mainPhoto" style="background-image: url(<?php echo $imgArr[0]; ?> );">
<div id="pageTitle">
<?php the_field('page_title'); ?>
</div>
</div>
<?php endif; ?>
