The oldest icon I can imagine would be the one written by St Luke of Our Lady holding the infant Jesus <img src="http://douglawrence.files.wordpress.com/2009/11/marymajor.jpg" /> regardless of whether you give any merit to the tradition that it was indeed done by him who was undoubtably the most well aquatinted man in the early Church with the Blessed Virgin Mary, excepting maybe St John the Evangelist, it is certainly an old picture and has been in the keeping of the St Mary Major for a very long time (~1700 years)