What´s the error in this syntax? - html

Question

I'm trying to do something with my Bootstrap .post-title class.
I want to put an element.style background on my post titles, which calls as a background, to the post featured image, for each post. I've already achieve this, but something went wrong and now isnt working. the only thing i know is must look something like this.
<div class="post-featured" style="background-image:url('upload/<?php the_post_thumbnail( 'wpbs-featured' ); ?>')">
but something in the syntax there is wrong, because it render this characters on HTML. whats going on?
')">
live example: WP Featured post image, as a div background

Answer 1

the_post_thumbnail returns an IMG html tag. So the generated code is
<div class="post-featured" style="background-image:url('upload/<img src="path/to/file.png">')">
Definitely not something that could work... You want the url only, so you should do this:
$thumb_id = get_post_thumbnail_id();
$thumb_url = wp_get_attachment_image_src($thumb_id,'wpbs-featured', true);
<div class="post-featured" style="background-image:url('<?= $thumb_url[0] ?>')">

Answer 2

this is the actual html snippet working.
<?php
$img = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), "full");
$img = $img[0];
?>
<div class="postfeatured" style="<?php if($img){echo 'background:url('.$img.');';} ?>">
<div class="page-header"><h1 class="single-title" itemprop="headline"><?php the_title(); ?></h1>
<h4>
<?php $mykey_values = get_post_custom_values('_aioseop_description');
foreach ( $mykey_values as $key => $value ) {
echo substr("$value",0 ,300); //This will display the first 150 symbols
}?>
</h4>
<hr>
<!-- Go to www.addthis.com/dashboard to customize your tools -->
<div class="addthis_native_toolbox"></div>
<hr>
</div>
</div>
live example: Design Thinking. Blog de Diseño Gráfico. Picoboero