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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)))
{
    TextWatermark watermark = new TextWatermark("Protected image", new Font("Arial", 8));
    watermark.HorizontalAlignment = HorizontalAlignment.Center;
    watermark.VerticalAlignment = VerticalAlignment.Center;
    watermark.RotateAngle = 45;
    watermark.SizingType = SizingType.ScaleToParentDimensions;
    watermark.ScaleFactor = 1;

    // Get all images from the first worksheet
    WatermarkableImageCollection images = doc.Worksheets[0].FindImages();

    // Add watermark to all found images
    foreach (WatermarkableImage image in images)
    {
        image.AddWatermark(watermark);
    }

    doc.Save();
}