Technology is a wonderful thing. In this case special x-ray technology was used to help reveal the underlying different image in this Rembrandt painting. Here is a little excerpt from an article on this subject from Huffington Post:
Scientists may be one step closer to revealing a hidden portrait behind a 380-year-old Rembrandt painting.
The masterpiece, "Old Man in Military Costume" by Dutch painter Rembrant Harmenszoon van Rijn, resides at the J. Paul Getty Museum in Los Angeles. Scientists had noticed the painting bears faint traces of another portrait beneath its surface. Researchers had previously probed the painting with infrared, neutron and conventional X-ray methods, but could not see the behind the top coat, largely because Rembrandt used the same paint (with the same chemical composition) for the underpainting and the final version.