WP - Show thumbnail of a specific page - php

Question

Trying to include a specific page on the frontpage of a WordPress Theme.
The overal HTML structure should be similar to this:
<div>
<h1>Om</h1>
<p>Content comes here</p>
</div>
<div style="background-image: url(...)">
</div>
At the moment it echos the content of the page successfully.
Here is how that part works with PHP:
<div class="omtekst col-md-6 half-content-wrapper">
<h1 class="semibold">Om</h1>
<?php
$my_id = 7;
$post_id_7 = get_post($my_id);
$content = $post_id_7->post_content;
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]>', $content);
echo $content;
?>
</div>
As mentioned earlier I want to include the thumbnail below this. This is where I am having problems.
However, I have started on the PHP to show how I am thinking. The thumbnail should be $image or similar.
<div class="ombilde col-md-6" style="background-image: url('<?php echo $image; ?>')">
Do you have a working solution for this?
Edit
My custom thumb which is not used with this image:
add_action( 'after_setup_theme', 'mytheme_custom_thumbnail_size' );
function mytheme_custom_thumbnail_size(){
add_image_size( 'frontpage_thumb', 500 ); // Unlimited height
}
Enabled post thumbnails:
if ( function_exists( 'add_theme_support' ) ) {
add_theme_support( 'post-thumbnails' );
add_theme_support( 'nav-menus' );
add_theme_support( 'widgets' );
}

Answer 1

to get the url of image you can use
$image = wp_get_attachment_image_src( get_post_thumbnail_id([ID_HERE]), [IMAGE SIZE HERE] );
then you can use
<div class="ombilde col-md-6" style="background-image: url('<?php echo $image[0]; ?>')">
to get image size string look at your theme for "add_image_size" function
and as pgk said post-thumbnails have to be enabled inside functions but i guess you already did this
edit:
[ID_HERE] replace with post id
[IMAGE SIZE HERE] you can test this with "full"

Answer 2

Use wp_get_attachment_url($id) for retrieving the URL of the desired post.
In your example:
$my_id = 7;
$post_id_7 = get_post($my_id);
$my_thumb = wp_get_attachment_url($my_id);
<div style="background-image: url('<?php echo $my_thumb; ?>')" class="omtekst col-md-6 half-content-wrapper">
<h1 class="semibold">Om</h1>
</div>

Answer 3

If you want to get post thumbnail, you can make this like that:
if ( has_post_thumbnail( $post_id_7->ID ) ) {
$image = get_the_post_thumbnail( $post_id_7->ID, $size, $args );
}
This returns image tag. You can find more info here Codex - get_the_post_thumbnail
Your theme must have ‘post-thumbnail’ support enabled.
This is the easiast way to attach image to your post.
And if you are in theLoop, you can use the_post_thumbnail() function, it directly prints thumbnail image:
<?php the_post_thumbnail( $size, $attr ); ?>
Codex - the_post_thumbnail()
If you want to print image as background, you must pit 'large' as $size.