In the five seasons that “Mad Men” has been on television that celebrated series set in the art-directed world of 1960s advertising has never marketed itself the way a ’60s ad man most certainly would have: by calling in a hotshot illustrator to do the job…Which explains how a 75-year-old illustrator living outside of Cambridge, England — highly regarded in his own country but little known in the United States — came to create the image that beginning this week will be emblazoned on buses, billboards, magazine pages, Web sites and TV.