Automatically add featured image

plugin.php

<?php

function _jpb_featured_image_setup(){
	define( 'JPB_THEME_REALLY_SUPPORTS_POST_THUMBNAILS', current_theme_supports('post-thumbnails') );
	if(!JPB_THEME_REALLY_SUPPORTS_POST_THUMBNAILS){
		add_theme_support( 'post-thumbnails' );
	}
}

add_action('after_setup_theme', '_jpb_featured_image_setup', 101);

function _jpb_featured_image_insert($content){
	if(JPB_THEME_REALLY_SUPPORTS_POST_THUMBNAILS || !has_post_thumbnail()){
		return $content;
	}
	// grab the post_thumbnail and add any html you need to it
	$image_html = get_the_post_thumbnail();
	$image_html = "<div class='feature'>$image_html</div>";
	return $image_html . $content;
}

add_filter( 'the_content', '_jpb_featured_image_insert' );