code: func Register(f httpimgPdf, urlStr, tp string) (info *gofpdf.ImageInfoType) {
  	info = f.GetImageInfo(urlStr)
  
  	if info != nil {
  		return
  	}
  
  	resp, err := http.Get(urlStr)
  
  	if err != nil {
  		f.SetError(err)
  		return
  	}
  
  	defer resp.Body.Close()
  
  	if tp == "" {
  		tp = f.ImageTypeFromMime(resp.Header["Content-Type"][0])
  	}
  
  	return f.RegisterImageReader(urlStr, tp, resp.Body)
  }
docs: // Register registers a HTTP image. Downloading the image from the provided URL
  // and adding it to the PDF but not adding it to the page. Use Image() with the
  // same URL to add the image to the page.