test.cs

public void AddWatermark(TXTextControl.ServerTextControl tx, string imagePath)
{
    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 = new TXTextControl.Image();
        image.FileName = imagePath;

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

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

        // calculate the vertical center location
        var pageHeight = (int)section.Format.PageSize.Height;
        var pageMarginsVertical = (int)(section.Format.PageMargins.Top +
            section.Format.PageMargins.Bottom);
        var imageHeight = image.Size.Height;

        int locationY = (pageHeight - imageHeight - pageMarginsVertical) / 2;

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