donaldG/functions.php

## functions.php
<?php
/**
 * This will be in the functions file of your theme
 */
// Add thumbnail support for site if it isn't there already
add_theme_support( 'post-thumbnails' );

// Set thumbnail size and cropping, this is the mobile image size I'm using
add_image_size( 'mobile', 800, 800, true );

// featured image source
function thumbnail_src( $thumb_name ) {
	global $post;
	if ( has_post_thumbnail() ) {
		$thumb = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), $thumb_name );
	}
	//You can add an else{} here if you like. Perhaps you have a placeholder image!
	$thumb_src = $thumb['0'];
	echo $thumb_src;
}

## header.php
<?php
/**
 * This will be in the header file of your theme
 * According to our functions file above we can pass a parameter for the thumbnail image name.
 * I've created a separate size for mobile devices
 */
//giving the body a background image but saying if WordPress is mobile to create a div we can apply some styles too
<body <?php body_class(); ?> style="background-image:url('<?php if(! wp_is_mobile()){ thumbnail_src('full');} else{ }?>');">
<?php if(wp_is_mobile()){?>
	<div id="mobile_background" style="background-image:url('<?php thumbnail_src('mobile');?>');"></div>
<?php } ?>

## style.css
/**
 * Standard fixed background for body
 * Mobile support for fixed backgrounds isn't great, hence why we're doing this
 */
body{
	background:no-repeat top left;
	background-size:cover;
	-o-background-size:cover;
	-moz-background-size:cover;
	-webkit-background-size:cover;
	background-attachment:fixed;
	font-family:'adelle-1', 'Georgia', serif;
}
/**
 * Mobile background div styles
 * Mobile support for fixed positioned items ain't bad
 */
#mobile_background{
	position:fixed;
	top:0;
	left:0;
	z-index:-1;
	width:100%;
	height:100%;
	background-attachment:scroll;
	background-size:cover;
	-o-background-size:cover;
	-moz-background-size:cover;
	-webkit-background-size:cover;
}
	<?php
	/**
	* This will be in the functions file of your theme
	*/
	// Add thumbnail support for site if it isn't there already
	add_theme_support( 'post-thumbnails' );

	// Set thumbnail size and cropping, this is the mobile image size I'm using
	add_image_size( 'mobile', 800, 800, true );

	// featured image source
	function thumbnail_src( $thumb_name ) {
	global $post;
	if ( has_post_thumbnail() ) {
	$thumb = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), $thumb_name );
	}
	//You can add an else{} here if you like. Perhaps you have a placeholder image!
	$thumb_src = $thumb['0'];
	echo $thumb_src;
	}
	/**
	* Standard fixed background for body
	* Mobile support for fixed backgrounds isn't great, hence why we're doing this
	*/
	body{
	background:no-repeat top left;
	background-size:cover;
	-o-background-size:cover;
	-moz-background-size:cover;
	-webkit-background-size:cover;
	background-attachment:fixed;
	font-family:'adelle-1', 'Georgia', serif;
	}
	/**
	* Mobile background div styles
	* Mobile support for fixed positioned items ain't bad
	*/
	#mobile_background{
	position:fixed;
	top:0;
	left:0;
	z-index:-1;
	width:100%;
	height:100%;
	background-attachment:scroll;
	background-size:cover;
	-o-background-size:cover;
	-moz-background-size:cover;
	-webkit-background-size:cover;
	}