Use this hook to add custom column to admin post/page list view.

    add_filter('manage_posts_columns', 'add_img_column');
    add_filter('manage_posts_custom_column', 'manage_img_column', 10, 2);
    
    function add_img_column($columns) {
    	$columns['img'] = 'Featured Image';
    	return $columns;
    }
    
    function manage_img_column($column_name, $post_id) {
    	if( $column_name == 'img' ) {
    		return get_the_post_thumbnail($post_id, 'thumbnail');
    	}
    }

Put this code in your `functions.php`