This snippet allows you to get the WordPress feature image and use it as a background.
Style it using the css class feature-bg. Remember to put the code around your content in your WordPress Template files.

The code will dynamically take the feature image thumbnail if the current post has one.

<?php if (has_post_thumbnail( $post->ID ) ): ?>
  <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
  <div class="feature-bg" style="background-image: url('<?php echo $image[0]; ?>')">
<?php endif; ?>