I saw someone do this to a neighbour and i thought it was really funny and not harmful. If you can then take the photos they have already put up inside their homes and photoshop them. Make their forehead bigger than what it already is, make someone smaller of taller than they are and stuff like that. If possible you can insert yourself in one of the photos. Then take the photoshopped photo and put it in front of their own photos inside their frame. You could also write a message behind the manipulated photo.