Final Code

// Image slideshow
    function image_slideshow() {
        $args = array('label' => __('Slides'), 'menu_position' => 5, 'public' => true, 'singular_label' => __('Slide'), 'supports' => array('title', 'editor', 'thumbnail'));
        register_post_type('slides', $args);
    }
    add_action('init', 'image_slideshow');

    // Custom columns
    function custom_columns($args) {
        $args = array('cb' => '<input type="checkbox" />', 'title' => __('Title'), 'thumbnail' => __('Thumbnail'), 'author' => __('Author'), 'categories' => __('Categories'), 'tags' => __('Tags'), 'date' => __('Date'));
        return $args;
    }
    add_filter('manage_posts_columns', 'custom_columns');

    // Image column
    function image_column($args, $post_id) {
        switch($args) {
            case 'thumbnail':
                echo the_post_thumbnail('admin-thumbnail');
            break;
        }
    }
    add_action('manage_posts_custom_column', 'image_column', 10, 2);