test.cs

public void AddWatermark(TXTextControl.TextControl tx, byte[] watermark)
{
    tx.PageUnit = TXTextControl.MeasuringUnit.Twips;

    foreach (TXTextControl.Section section in tx.Sections)
    {
        // remove existing headers for demo purposes
        section.HeadersAndFooters.Remove(TXTextControl.HeaderFooterType.All);

        // add new header for each section
        section.HeadersAndFooters.Add(TXTextControl.HeaderFooterType.Header);
        TXTextControl.HeaderFooter hf =
            (section.HeadersAndFooters.GetItem(TXTextControl.HeaderFooterType.Header));

        // add the watermark
        TXTextControl.Image image;

        using (MemoryStream ms = new MemoryStream(watermark, 0, watermark.Length, writable: false, publiclyVisible: true))
        {
            image = new TXTextControl.Image(ms);

            hf.Images.Add(
                image,
                1,
                new System.Drawing.Point(0, 0),
                TXTextControl.ImageInsertionMode.FixedOnPage |
                TXTextControl.ImageInsertionMode.BelowTheText);
        }

        // Calculate the horizontal center location
        int locationX = (int)((section.Format.PageSize.Width - image.Size.Width -
            (section.Format.PageMargins.Left + section.Format.PageMargins.Right)) / 2);

        // Calculate the vertical center location
        int locationY = (int)((section.Format.PageSize.Height - image.Size.Height -
            (section.Format.PageMargins.Top + section.Format.PageMargins.Bottom)) / 2);

        // set the location
        image.Location = new System.Drawing.Point(locationX, locationY);
    }
}