Solution - 1

First, you need to download [[LINK href="http://code.google.com/p/timthumb/"]]TimThumb [[/LINK]], and put it into your theme folder. We use it to resize picture. Add the following code to functions.php:

add_action('wp_head', 'insert_post_bg');

function insert_post_bg() {
	if (!is_single()) return;
	
	global $post;

	$bg = get_post_meta($post->ID, 'background', true);
	$bg = str_replace(get_bloginfo('url'), '', $bg);
	if (!empty($bg)) {
	?>
		<style type="text/css" media="screen">
		body{background: url(<?php echo get_stylesheet_directory_uri(); ?>/timthumb.php?w=1920&zc=1&src=<?php echo $bg; ?>) fixed no-repeat;}
		</style>
	<?php 
	}
}

body{background: url(<?php echo get_stylesheet_directory_uri(); ?>/timthumb.php?w=1920&zc=1&src=<?php echo $bg; ?>) fixed no-repeat;}

Solution - 2

The PHP code that Rilwis and Oleg gave you is great, but you'll need to use Javascript to resize the image to fit the browser window (since PHP is server side and has no way to get the window size). Edit Rilwis' insert_post_bg function like this:

function insert_post_bg() {
	if (!is_single()) return;

	global $post;

	$bg = get_post_meta($post->ID, 'background', true);
	$bg = str_replace(get_bloginfo('url'), '', $bg);
	if (!empty($bg)) {
	?>
		<style type="text/css" media="screen">
		body{background: url(<?php echo get_stylesheet_directory_uri(); ?>/timthumb.php?w=1920&zc=1&src=<?php echo $bg; ?>) fixed no-repeat;}
		</style>
		<script type="text/javascript">
		jQuery(function(){
			jQuery("body").css('background-image','url(<?php echo get_stylesheet_directory_uri(); ?>/timthumb.php?w=' +screen.width+ '&zc=1&q=90&src=<?php echo $bg; ?>)');
		});
		</script>
	<?php 
	}
}

Solution - 3

You can use CSS3 attributes to resize background images. Then use a jQuery fix for browsers that dont support CSS3 attributes, i.e. Internet Explorer 8 and below

function insert_post_bg() {

	if (!is_single()) return;

	global $post;

	$bg = get_post_meta($post->ID, 'background', true);

	if (!empty($bg)) {

	?>

		<style type="text/css" media="screen">

		body{background: url(<?php echo $bg; ?>);
                        background-repeat:no-repeat;
                        background-position:center center;
                        background-attachment:fixed;
                        -o-background-size: 100% 100%, auto;
                        -moz-background-size: 100% 100%, auto;
                        -webkit-background-size: 100% 100%, auto;
                        background-size: 100% 100%, auto;
                       }

		</style>

		<script type="text/javascript">

		$(document).ready(function() {
                
                $("body").append('<img class="bg-resize" src="<?php echo $bg; ?>" />');
                $("img.bg-resize").hide();

		var $winwidth = $(window).width();
		$("img.bg-resize").attr({
			width: $winwidth
		});
		$(window).bind("resize", function(){
			var $winwidth = $(window).width();
			$("img.bg-resize").attr({
				width: $winwidth
			});
		 });

	        });

		</script>

	<?php 

	}

}

Solution - 4

1) enable featured iamge for page and post (paste this code into functions.php) add_theme_support( 'post-thumbnails' ); add_theme_support( 'post-thumbnails', array( 'post' ) ); add_theme_support( 'post-thumbnails', array( 'page' ) );

Solution - 5

A way simpler way to do all of this: [[LINK href="http://codecanyon.net/item/custom-backgrounds-for-wordpress/121983?ref=mordauk"]]Custom Backgrounds Plugin from Code Canyon[[/LINK]]