How to use featured image as background image in wordpress - php

Question

mysite.com
I would like to be able to allow the user of this twentytwelve child theme to be able to upload a featured image to a page (not post) and have that dynamically become the background image (positioned basically the way it is now-see thesite.com)
I think it would need a dynamic url (like wp_get_attachment_thumb_url ), though I'm not using a loop. (was thinking that function has to be used inside a loop.
I am using the code below as a template part. This is what is generating the content on the front page. So basically, I need to get this featured image and use it as a background image for a templage part, and not inside a loop.
<?php $page = get_page_by_title ( 'Welcome' ); ?>
<div id="welcome-intro" class="clear">
<?php if ( get_the_post_thumbnail ( $page->ID ) ) : ?>
<figure class="entry-page-image">
<?php echo get_the_post_thumbnail ( $page->ID, 'full'); ?>
</figure>
<?php endif; ?>
<article id="intro-elements">
<div class="welcome-entry-content">
<?php echo apply_filters( 'the_content', $page->post_content); ?>
</div>
</article>
</div>
/* >>>>>>>>>>>>>>MODIFED<<<<<<<<<<<<<<< */
<?php
// welcome message ?>
<?php $page = get_page_by_title ( 'Welcome' ); ?>
<?php if ( $background = wp_get_attachment_image_src( get_post_thumbnail_id( $page->ID ), 'full' ) ) : ?>
<div style="background: url('<?php echo $background[0]; ?>') top right no-repeat; height:400px;" id="welcome-intro" class="clear">
<?php endif; ?>
<article id="intro-elements">
<div class="welcome-entry-content">
<?php echo apply_filters( 'the_content', $page->post_content); ?>
</div>
</article>
</div>`

Answer 1

You can do it in this way by using css
$background = wp_get_attachment_image_src( get_post_thumbnail_id( $page->ID ), 'full' );
<style>
body{
background-image: url('<?php echo $background[0]; ?>');
}
</style>