onepage sections with thumbnails as background

I’m creating a onepage. Pages are listed underneath each other and are display fullscreen. Each section should have the posts featured image as background. With my code in index.php it doesn’t work though:

<?php
                    $args = array(
                        'sort_order' => 'ASC',
                        'sort_column' => 'menu_order', //post_title
                        'hierarchical' => 1,
                        'exclude' => '',
                        'child_of' => 0,
                        'parent' => -1,
                        'exclude_tree' => '',
                        'number' => '',
                        'offset' => 0,
                        'post_type' => 'page',
                        'post_status' => 'publish'
                    );
                    $pages = get_pages($args);
                    //start loop
                    $count = 0;
                    foreach ($pages as $page_data) {
                    $count++;
                        $content = apply_filters('the_content', $page_data->post_content);
                        $title = $page_data->post_title;
                        $slug = $page_data->post_name;
                        $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'bg' );
                    ?>               

                    <section id="page-<?php echo $count; ?>" style="background-image: url('<?php echo $thumb['0'];?>')" class='page-section <?php echo "$slug" ?>'><a name='<?php echo "$slug" ?>'></a>
                        <div class="wrap">
                                <h1><?php echo "$title" ?></h1>
                                <?php echo "$content" ?>
                        </div>
                    </section>
                    <?php } ?>

When I check it in the browser with the inspector, I get the following code:
background-image: url('');