Images & MarkUp ( iTextSharp)

Hello,

I am new to iTextSharp. Here is what I need to do. Take a Tiff image and make a PDF document. I have done that. :)Now I need to mark certain part of the Image which may contain words in a polygon/rectangle. How do I do this?

I tried doing

                    cb.SetRGBColorFillF(0f, 255f, 0f);
                    cb.MoveTo(70f, 30f);
                    cb.LineTo(78f, 60f);
                    cb.LineTo(80f, 90f);
                    cb.SetRGBColorFillF(255f, 0f, 0f);
                    cb.ClosePathFillStroke();
                    img.SetAbsolutePosition(0, 0);
                    cb.AddImage(img);
                    document.NewPage();

but hasn't helped. What am I missing?  or am I on the right track?

Thanks