gistfile1.txt

// GET FEATURED IMAGE
function ST4_get_featured_image($post_ID) {
  $post_thumbnail_id = get_post_thumbnail_id($post_ID);
	if ($post_thumbnail_id) {
		$post_thumbnail_img = wp_get_attachment_image_src($post_thumbnail_id, 'featured_preview');
		return $post_thumbnail_img[0];
	}
}

// ADD NEW COLUMN
function ST4_columns_head($defaults) {
  $defaults['featured_image'] = 'Featured Image';
	return $defaults;
}

// SHOW THE FEATURED IMAGE
function ST4_columns_content($column_name, $post_ID) {
	if ($column_name == 'featured_image') {
		$post_featured_image = ST4_get_featured_image($post_ID);
		if ($post_featured_image) {
			echo '<img src="' . $post_featured_image . '" />';
		}
	}
}
add_filter('manage_posts_columns', 'ST4_columns_head');
add_action('manage_posts_custom_column', 'ST4_columns_content', 10, 2);