There is an item in the New York Times today about how infrared photography revealed a portrait of a man with a mustache painted on the canvas before Picasso painted Woman Ironing. The painting was recently cleaned and the ghost portrait is quite clear.
And the NYT employs some nifty technology to illustrate it: Rotate the canvas upside down, and move your mouse over the surface to reveal the image below. They also give some clues about who the man in the portrait could be.