code: func RegisterReader(fpdf *gofpdf.Fpdf, imgName string, options gofpdf.ImageOptions, r io.Reader) (info *gofpdf.ImageInfoType) {
  	var err error
  	var img image.Image
  	var buf bytes.Buffer
  	if fpdf.Ok() {
  		if options.ImageType == "tiff" || options.ImageType == "tif" {
  			img, err = tiff.Decode(r)
  			if err == nil {
  				err = png.Encode(&buf, img)
  				if err == nil {
  					options.ImageType = "png"
  					info = fpdf.RegisterImageOptionsReader(imgName, options, &buf)
  				}
  			}
  		} else {
  			err = fmt.Errorf("expecting \"tiff\" or \"tif\" as image type, got \"%s\"", options.ImageType)
  		}
  		if err != nil {
  			fpdf.SetError(err)
  		}
  	}
  	return
  }
docs: // RegisterReader registers a TIFF image, adding it to the PDF file but not
  // adding it to the page. imgName specifies the name that will be used in the
  // call to Image() that actually places the image in the document. options
  // specifies various image properties; in this case, the ImageType property
  // should be set to "tiff". The TIFF image is a reader from the reader
  // specified by r.