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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (SlidesDocument doc = Document.Load<SlidesDocument>(Utilities.MapSourceFilePath(FilePath)))
{
    SlidesSlide slide = doc.Slides[0];
    slide.ImageFillFormat.BackgroundImage = new SlidesWatermarkableImage(File.ReadAllBytes(@"D:\background.png"));
    slide.ImageFillFormat.TileAsTexture = true;
    slide.ImageFillFormat.Transparency = 0.5;
    doc.Save();
}