This plugin displays featured Images on edit.php.

• A new column “featured image” is added on edit.php.
• If it exists, a featured image is shown on each post row.
• The size of images is 150×150.
• This plugin does not affect front end.

The column “Featured Image” (アイキャッチ画像 in Japanese) is displayed in this picture. When posts have no featured image, nothing will be shown.

function show_featured_image_post_column( $defaults ) {
       $defaults['featuredimage'] = __('Featured Image') ;
    	return $defaults;
}
add_filter('manage_posts_columns', 'show_featured_image_post_column');

function edit_featured_images($column_name, $id) {
    if( $column_name == 'featuredimage' && has_post_thumbnail()) {
        the_post_thumbnail('thumbnail');
    }
}
add_action('manage_posts_custom_column', 'edit_featured_images', 10, 2);

You can download the plugin → Featured Image on Edit.php