How to automatically pull a featured image up into the post itself

I’m trying to automatically pull a post’s featured image up into the post itself.

I’ve added this at the top of functions.php

if ( function_exists( 'add_theme_support' ) ) {
  add_theme_support( 'post-thumbnails' );
}

And this:

<?php
if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
  the_post_thumbnail();
}
?>

in content-extensions.php above:

<?php
    				$more = 0;       // Set (inside the loop) to display content above the more tag.

    				the_content( __( 'Read more', 'attitude' ) );

It’s not working, and I’d be grateful if someone could point me in the right direction.