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You can use the following code to align an image at the center using NPOI in C#:

// Get the cell style
ICellStyle style = workbook.CreateCellStyle();

// Set the horizontal alignment to center
style.Alignment = HorizontalAlignment.Center;

// Set the vertical alignment to center
style.VerticalAlignment = VerticalAlignment.Center;

// Create the drawing patriarch to work with
IDrawing drawing = sheet.CreateDrawingPatriarch();

// Create the anchor for the image
IClientAnchor anchor = drawing.CreateAnchor(0, 0, 0, 0, cellIndex, rowIndex, cellIndex, rowIndex);

// Add the image to the drawing patriarch
IDrawing patriarch = sheet.CreateDrawingPatriarch();
IPicture picture = patriarch.CreatePicture(anchor, pictureIndex);

// Set the cell style for the cell
cell.CellStyle = style;

In this code, workbook is your NPOI workbook object, sheet is the worksheet where you want to add the image, cellIndex and rowIndex are the indexes of the cell where you want to add the image, and pictureIndex is the index of the image in the workbook's picture collection.

Note that this code assumes that you already have a picture added to the workbook's picture collection. If you haven't added the picture yet, you can do so using the following code:

IPictureData data = workbook.AddPicture(imageBytes, PictureType.PNG);
int pictureIndex = workbook.GetPictureIndex(data);

In this code, imageBytes is a byte array containing the image data in PNG format. You can replace PictureType.PNG with the appropriate image type if your image is in a different format.