add_filter('manage_posts_columns', 'img_kolon_ekle');
add_filter('manage_posts_custom_column', 'img_kolon_ayar', 10, 2);
function img_kolon_ekle($columns) {
    $columns['img'] = 'Öne Çıkan Görsel';
    return $columns;
}
function img_kolon_ayar($column_name, $post_id) {
    if( $column_name == 'img' ) {
        echo get_the_post_thumbnail($post_id, 'thumbnail');
    }
    return $column_name;
}