Insert featured image into post content - php

Question

I am using a wordpress theme for my website and I'm trying to alter the php so that the featured image is displayed within the post content instead of before or after (the top of the post or the bottom)
Here's the original code:
<?php if ( ( has_post_thumbnail( $post_id ) || '' != get_post_meta( $post_id, 'Thumbnail', true ) ) && 'on' == et_get_option( 'origin_thumbnails' ) ) { ?>
<div class="post-thumbnail">
<?php
if ( has_post_thumbnail( $post_id ) ) the_post_thumbnail( 'full' );
else printf( '<img src="%1$s" alt="%2$s" />', esc_attr( get_post_meta( $post_id, 'Thumbnail', true ) ), the_title_attribute( array( 'echo' => 0 ) ) );
?>
</div> <!-- end .post-thumbnail -->
<?php } ?>
<?php the_content(); ?>
Here's how I tweaked it in my child theme:
<?php the_content(); ?>
<?php if ( ( has_post_thumbnail( $post_id ) || '' != get_post_meta( $post_id, 'Thumbnail', true ) ) && 'on' == et_get_option( 'origin_thumbnails' ) ) { ?>
<div class="post-thumbnail">
<?php
if ( has_post_thumbnail( $post_id ) ) the_post_thumbnail( 'full' );
else printf( '<img src="%1$s" alt="%2$s" />', esc_attr( get_post_meta( $post_id, 'Thumbnail', true ) ), the_title_attribute( array( 'echo' => 0 ) ) );
?>
</div>
I'm not very fluent in php and all that accomplished was moving the featured image to the bottom of the post :-/. I posted to my theme's forum and was given the following code which didn't seem to do anything.
add_filter( 'the_content', insert_featured_image, 20 );
function insert_featured_image( $content ) {
$content = preg_replace( "/<\/p>/", "</p>" . get_the_post_thumbnail($post->ID, 'post-single'), $content, 1 );
return $content;
}
Can anyone point me in the right direction?
TIA!

Answer 1

What is the reasoning behind attempting to move the featured image into the content? The Wordpress codex gives the following as an example:
// check if the post has a Post Thumbnail assigned to it.
if ( has_post_thumbnail() ) {
the_post_thumbnail();
}
the_content();
Basically, the_post_thumbnail(); will display the image that was selected as the featured image while the_content(); is simply displaying what was inputted into the wysiwyg for that post or page.
If you're attempting to do this so that it matches a design or layout, you might want to look at applying addition markup and styles to this area. If you require text before and after the featured image you should look into custom fields.