Adding Post Thumbnails

I’m using the recent posts as my landing page, I would like to add the thumbnail to the excerpt. I’ve added to the function.php, but not certain if there is something to be added to another script, the ‘featured image’ field is not showing in edit posts either. My current image is embedded into the post

/** Add theme support for Feed Links. */
	add_theme_support( 'automatic-feed-links' );

	/** Add theme support for Custom Background. */
	add_theme_support( 'custom-background', array( 'default-color' => 'fff' ) );

	/** Set content width. */
	chiron_set_content_width( 640 );

        <strong>/** Add theme support for Post Thumbnail. */
        add_theme_support( 'post-thumbnails' );</strong>

	/** Add custom image sizes. */
	add_action( 'init', 'chiron_add_image_sizes' );

[Please post code & markup between backticks or use the code button. Your posted code may now have been permanently damaged by the forum’s parser.]