// 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();
}
}