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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (Document doc = Document.Load(Utilities.MapSourceFilePath (PptFilePath)))
{
    // Use path to the image as constructor parameter
    using (ImageWatermark watermark = new ImageWatermark(@"D:\watermark.jpg"))
    {
        // Add watermark to the document
        doc.AddWatermark(watermark);

        doc.Save();
    }
}