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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (CellsDocument doc = Document.Load<CellsDocument>(Utilities.MapSourceFilePath (FilePath)))
{
    using (ImageWatermark watermark = new ImageWatermark(@"D:\logo.gif"))
    {
        watermark.HorizontalAlignment = HorizontalAlignment.Center;
        watermark.VerticalAlignment = VerticalAlignment.Center;
        watermark.RotateAngle = 90;
        watermark.SizingType = SizingType.ScaleToParentDimensions;
        watermark.ScaleFactor = 0.5;
        doc.Worksheets[0].AddWatermarkAsBackground(
              watermark,
              doc.Worksheets[0].ContentAreaWidthPx /*set background width*/,
              doc.Worksheets[0].ContentAreaHeightPx /*set background height*/);
    }

    doc.Save();
}