<?php if (get_sub_field('background_colour') == 'image') : ?>
  <?php if (get_sub_field('background_image')) : ?>
    <?php $attachment_id = get_sub_field('background_image'); ?>
    <?php $image = wp_get_attachment_image_src($attachment_id, 'full'); ?>
    style="background: url('<?php echo $image[0]; ?>') no-repeat center; background-size: cover;"
  <?php endif; ?>
<?php endif; ?>