if you blackmail her for a while and then you bust her, more than likely she will also tell your dad that you were blackmailing her. He'll probably be pissed that you didn't tell him right away.
Here is what I would do...I'd make a hundred copies of the photo and plaster them all over the house. The shear panic she displays oughta be priceless! Time it right so that she only has like 5 minutes to try and get them all before your dad walks in!!