code: func (captcha *CaptchaImage) SaveImage(w io.Writer, imageFormat ImageFormat) error {
  
  	if imageFormat == ImageFormatPng {
  		return png.Encode(w, captcha.nrgba)
  	}
  	if imageFormat == ImageFormatJpeg {
  		return jpeg.Encode(w, captcha.nrgba, &jpeg.Options{Quality: 100})
  	}
  	if imageFormat == ImageFormatGif {
  		return gif.Encode(w, captcha.nrgba, &gif.Options{NumColors: 256})
  	}
  
  	return errors.New("not supported image format")
  }
docs: //保存图片对象