I don't get this blackmail. I know Sasaki doesn't want pictures of her FUCKING RAPE on the internet, but they're extremely incriminating things that would immediately cause huge problems for Teacher's Son. Dude's in possession of violent child porn right now and the cops are actively investigating social media shit around the case, doing this puts him in incredible danger.
Emotionally there's leverage, sure, but the thing that would happen if that uncensored photo got out would be a complete change in how his father's case was looked at in the public eye, and everyone immediately going ooooh, that's what happened and the two people he's blackmailing seen as tragic victims rather than a murderer and a witness.