<?php
$image = get_field( 'image' );

if ( ! empty( $image ) ) {
    // thumbnail
    $size  = 'medium';
    $thumb = $image['sizes'][ $size ];
} else {
    $thumb = '/wp-content/uploads/2019/01/placeholder.jpg';
} ?>

<div class="lozad" data-background-image="<?= $thumb ?>" style="background-repeat: no-repeat; background-position: center; background-size: cover;"></div>