InsertWatermarkImages.cs

private void InsertWatermarkImages()
{
    // remove all existing watermarks
    RemoveWatermarkImages(textControl1);

    foreach (TXTextControl.Page page in textControl1.GetPages())
    {
        // create a new watermark image
        Bitmap bmp = CreateDraftImage();

        // create a new TXTextControl.Image object and mark
        // as watermark
        TXTextControl.Image img = new TXTextControl.Image(bmp);
        img.Name = "Watermark";
        img.Sizeable = false;
        img.Moveable = false;

        // calculate the location to center the image
        Point pImageLocation = new Point(
            (page.Bounds.Width / 2) - (PixelToTwips(bmp.Size.Width)) / 2,
            (page.Bounds.Height / 2) - (PixelToTwips(bmp.Size.Height)) / 2);

        // add the image to the page
        textControl1.Images.Add(
            img,
            page.Number,
            pImageLocation,
            TXTextControl.ImageInsertionMode.BelowTheText);
    }
}