I'm being blackmailed

Okay so I'm 15 and I used to show my butt with shorts on and stuff and this guy stalked me and I didn't know until a couple weeks ago, he posted the inappropriate pictures and followed kids at my school and everyone knows but they deleted the pictures and is saying he will post again if I don't show my but hole and I don't want to tell my dad because he will go crazy he scares me what do I do. I would go to the police but they would get my parents involved help me And my dad will hit me with the belt for this