Make featured image background on WP posts and most recent post's featured image background on home page.

add_background_image.php

// Custom Background Images
// Use featured image on posts 
// Use the most recent post's featured image on home page (or default background image)

function add_background_image() {

    global $post;
    		
	if (is_admin()) return;

	// get the global background image
	$page_bgimg_url = get_background_image();

	// if home then override default with most recent post 
	if ( is_front_page() || is_home() ) {
	
		$posts = get_posts( array('posts_per_page'=>1,'orderby'=>'post_date','order'=>'desc') );
		echo $posts[0]->ID;
		
		if ($posts) {		
			$page_bgimg = wp_get_attachment_image_src( get_post_thumbnail_id( $posts[0]->ID ), 'full' );
			$page_bgimg_url = $page_bgimg[0]; // this returns just the URL of the image
		}
		
	} 
	
	// if single and single active
	if ( is_single() ) {

    	// check to see if the theme supports Featured Images, and one is set
    	if (has_post_thumbnail( $post->ID )) {
            
        	// specify desired image size in place of 'full'
        	$page_bgimg = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
        	$page_bgimg_url = $page_bgimg[0]; // this returns just the URL of the image

    	} 
    		
	}
    	
    // get background image for a category 
    if ( is_archive() && function_exists('category_image_src') ) {

    	if ( category_image_src() != '' ) {
    		$page_bgimg_url = category_image_src();
    	}
    } 

	// write out css
	
	if ($page_bgimg_url != '') {

		echo '
		<style type="text/css" id="custom-background-css-override">
        	body { 
        		background: url(' . $page_bgimg_url . ') no-repeat center center fixed; 
  				-webkit-background-size: cover;
  				-moz-background-size: cover;
  				-o-background-size: cover;
  				background-size: cover; 
        	}
    	</style>';
    }

}

add_action('wp_head' , 'add_background_image' , 9999);