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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)))
{
    doc.Worksheets[0].Charts[0].ImageFillFormat.BackgroundImage = new CellsWatermarkableImage(File.ReadAllBytes(@"D:\test.png"));
    doc.Worksheets[0].Charts[0].ImageFillFormat.Transparency = 0.5;
    doc.Worksheets[0].Charts[0].ImageFillFormat.TileAsTexture = true;
    doc.Save();
}