Thanks <itsnotabug> I read up a bit on watermarks etc and using the "UnderConent". My code now looks like this for anyone interested. I read the original PDF in, scale the image, find the last page and add the image, then save the pdf again to a different filename. Brilliant!

Thanks!

            PdfReader reader = new PdfReader(myoriginalPDFFileName);
           
            PdfStamper stamper = new PdfStamper(reader, new FileStream(
                mynewPDFFileName, FileMode.Create));
                       
            Single x=0;
            Single y=0;
            iTextSharp.text.Rectangle rect = reader.GetPageSizeWithRotation(1);
            iTextSharp.text.Image img = iTextSharp.text.Image.GetInstance(myImageName);
            if (img.Width > rect.Width || img.Height > rect.Height)
            {
                img.ScaleToFit(rect.Width, rect.Height);
                x = (rect.Width - img.ScaledWidth) / 2;               
                //y = (rect.Height - img.ScaledHeight) / 2;
                y = 0; //Set at bottom
            }
            else
            {
                x = (rect.Width - img.Width) / 2;
                y = (rect.Height - img.Height) / 2;
            }
            img.SetAbsolutePosition(x, y);

            int pageCount = reader.NumberOfPages;
            PdfContentByte underContent = stamper.GetUnderContent(pageCount);

            underContent.AddImage(img);

            // flatten form fields and close document
            stamper.FormFlattening = true;
            stamper.Close();