Add a background image via a custom field and no theme hacking.

custom-image-no-theme-hack.php

<?php 
add_filter( 'the_content', function( $content ){
  
  // Just pretend you have a custom metabox that saves the ID for you.
  $image_id = get_post_meta( 'custom_image', $post->ID );
  $image_src = ''; // instantiate variable
  
  if ( !empty( $image_id ) ){
    $image_src = wp_get_attachment_image_src( $image_id, 'large');
  }
  
  if ( !empty( $image_src ) ){
    return "<div class='has-bg-image' style='background-image: url(\" {$image_src} \" );'>" . $content . "</div>";
  }
  
  // fallback for if there is no image
  return $content;
  
}, 10, 1 );