Below is how you would use the featured image as a background image (also with an if statement so you can have a fallback image).
<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' );
$image = $image[0]; ?>
<?php else :
$image = get_bloginfo( 'stylesheet_directory') . '/images/default_cat_img.jpg'; ?>
<?php endif; ?>
<div id="this-is-whatever-or-not" style="background-image: url('<?php echo $image; ?>')" >