I thought it was just a photo in his house and he wrote a message on some paper on top of the photo. If you write quite hard the impression goes through and by doing what she did, rubbing over it you can see the message indented on the paper.
If you look up The Birmingham Six and ESDA test it shows how they proved the police tampered with statements to incriminate them. ESDA is a sort of high tech version of rubbing over the paper with a pencil.
Another Tory MP crosses the floor
Petition on Why the old state pension is £200 pm less than the new