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' For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
Using doc As CellsDocument = Document.Load(Of 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()