Sure in that case something that could be considered computer forensics was useful for obtaining evidence. A key difference is that they obtained evidence that was in itself incriminating, ie: the photographs. If somebody has pictures of them raping kids and the police use computer forensics to obtain the pictures, then the pictures are evidence. If somebody has pictures of somebody else raping kids, and the police use computer forensics to obtain them, they are not going to be able to prove beyond a shadow of a doubt that the person knowingly possessed the pictures, even if the forensics indicate that they did.