Adding watermark to photos | https://docs.conholdate.com/net/adding-watermark-to-photos/

adding-watermark-to-photos.cs

TiffImageLoadOptions loadOptions = new TiffImageLoadOptions();
// Constants.InImageTiff is an absolute or relative path to your document. Ex: @"C:\Docs\image.tiff"
using (Watermarker watermarker = new Watermarker(Constants.InImageTiff, loadOptions))
{
    // Initialize text or image watermark
    TextWatermark watermark = new TextWatermark("Test watermark", new Font("Arial", 19));

    // Add watermark to the first frame
    TiffImageWatermarkOptions options = new TiffImageWatermarkOptions();
    options.FrameIndex = 0;

    watermarker.Add(watermark, options);
    watermarker.Save(Constants.OutImageTiff);
}

Read the complete article: https://docs.conholdate.com/net/adding-watermark-to-photos/