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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>(@"D:\test.xlsx"))
{
    using (ImageWatermark watermark = new ImageWatermark(@"D:\logo.gif"))
    {
        doc.AddWatermarkAsBackground(watermark);
    }
    doc.Save();
}