I figured it out myself few hrs. later. All i had to was, load an image into a chunk and then load the chunk into a phrase for header. Below is the code snippet.
  IText.Image image = IText.Image.GetInstance(@"C:\ImageLib\Header.jpg");
    IText.Chunk ck = new Chunk(image, 0F, 0F);
    HeaderFooter header = new HeaderFooter(new Phrase(ck), false);

Thanks for all the minds that thought abt. a solution to my question.