Actually, it turned out to be pretty simple. I just replaced the first code with this:
<div id="mainPhoto" style="background-image: url(<?php the_field('background_image'); ?>);">
<div id="pageTitle">
<?php the_field('page_title'); ?>
</div>
</div>
I’ve tried something like this before, and it didn’t work. This time it did. The first bit of code was copied/adapted from a previous project (where it works) because trying it this way didn’t work in the previous project.