You can post things anonymously, so it's less likely that he would get immediately caught by posting it. Sure she could pin it on him, but with no proof he did it, not much would happen. And once something is on the internet, it never leaves. By the time it gets taken down, the creep shots he took of her would be seen by who knows how many people, saved by most of them, and shared around to even more.
As much as I hate NTR, the "blackmail through creep shots" angle makes more sense than a lot of other plots