featured image inside post

@PatJ’s answer affords the most control over the placement of the image but involves editing the theme, which may or may not be wise, or possible, depending on the circumstance, so an alternative is to add a filter to the_content.

function add_thumb_wpse_100914($content) {
    // check that we are on a 'single' post display and...
    // check if the post has a Post Thumbnail assigned to it.
    if ( is_single() && has_post_thumbnail() ) { 
      $content = get_the_post_thumbnail(null,'full').$content;
    } 
    return $content;
}
add_filter('the_content','add_thumb_wpse_100914');