Wordpress Featured Image Not Displaying - php

Question

I am trying to show a small version of the featured image as a post thumbnail on my main page (index.php). For this I am implementing it as the background-image of a div. Unfortunately the code (which had been working before) has stopped working now and I cannot find a reason why. I have been looking everywhere for a solution, but I just cant figure it out.
I am using the following code but unfortunately it doesnt return anything:
<?php $post_image_id = get_post_thumbnail_id($post_to_use->ID);
if ($post_image_id) {
$thumbnail = wp_get_attachment_image_src( $post_image_id, 'post-thumbnail', false);
if ($thumbnail) (string)$thumbnail = $thumbnail[0];
} ?>
<?php if (has_post_thumbnail()) : ?>
<div class="post_image_crop" style="background: url('<?php echo $thumbnail; ?>')">
</div>
<?php endif; ?>
You can have a look at it here: oliverprenzel.com
The weird thing is, I have done exactly the same thing on my single.php where it still does work.
<?php $post_image_id = get_post_thumbnail_id($post_to_use->ID);
if ($post_image_id) {
$thumbnail = wp_get_attachment_image_src( $post_image_id, 'post-thumbnail', false);
if ($thumbnail) (string)$thumbnail = $thumbnail[0];
} ?>
<?php if (has_post_thumbnail()) : ?>
<div class="post_image_bg" style="background: url('<?php echo $thumbnail; ?>'); background-size: 100% !important;">
</div>
<?php endif; ?>
You can have a look at it working here: oliverprenzel.com/headmagazine/
Does anyone have an idea what might be causing this?
Edit for Claudiu:
This is the loop I am trying to get the image in:
<div class="wrapper">
<?php $post_image_id = get_post_thumbnail_id( $post_id );
if ($post_image_id) {
$thumbnail = wp_get_attachment_image_src( $post_image_id, 'post-thumbnail', false);
if ($thumbnail) (string)$thumbnail = $thumbnail[0];
} ?>
<!-- post loop prev -->
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<a href="<?php the_permalink() ?>">
<div class="post_frame">
<div class="post_image_crop" style="background: url('<?php echo $thumbnail; ?>')">
</div>
<div class="prev_det">
<div class="prev_det_center">
<h1>
<?php the_title(); ?>
</h1>
<p><?php echo get_the_category_list(', '); ?></p>
</div>
</div>
<div class="prev_det_fold"></div>
</div>
</a>
<?php endwhile; ?>
<?php else: ?>
<?php endif; ?>
</div>

Answer 1

If you want to get each posts thumbnail, then you need to move
<?php $post_image_id = get_post_thumbnail_id( $post_id );
if ($post_image_id) {
$thumbnail = wp_get_attachment_image_src( $post_image_id, 'post-thumbnail', false);
if ($thumbnail) (string)$thumbnail = $thumbnail[0];
} ?>
inside the loop and change it to:
<?php $post_image_id = get_post_thumbnail_id( get_the_ID() );
if ($post_image_id) {
$thumbnail = wp_get_attachment_image_src( $post_image_id, 'post-thumbnail', false);
if ($thumbnail) (string)$thumbnail = $thumbnail[0];
} ?>
My guess is that $post_image_id is returning null because you don't have a featured image for your home page and even if you had, the same image will be displayed for each post.

Answer 2

I have a strong suspicion that in your first line $post_to_use->ID returns null. When the argument in get_post_thumbnail_id() returns null then the current post id is used by default. This is why it is working in single.php, because there you have a post loaded.
On frontpage you don't. So you have to check which post id you want, manually enter it or do a loop.
Try replacing get_post_thumbnail_id($post_to_use->ID) with get_post_thumbnail_id(5) where 5 is the post id that you want.