Image Validation

if($_FILES['logo']['name']!='') {                       
                        $img = explode('.', $_FILES['logo']['name']);
                        $img_path = $img_dir.'/logo'.$img[1];
                        $extension = strtolower($img[1]);
                        if(in_array($extension , array('jpg','jpeg', 'gif', 'png', 'bmp'))){
                                move_uploaded_file($_FILES['logo']['tmp_name'], $img_path);
                                update_site_option('rt_blog_thumb', $blog_thumb);
                        }
                        else echo 'Invalid file';
                }