You have choices--

You can keep up the fantasy relationship with whomever is at the other end of the keyboard because you are getting your needs met from it. Are you so scared of living an authentic life that you would rather live a fantasy one? Still, if you choose to continue, I'd have the real discussion with him about how you both agree this is just a fantasy and you are never really going to be together.

Or you can be honest with him about your need to really know who he is. Tell him that you will not judge him for sending you a photo of someone else. You want to Skype with this real person and if he will not, then you will block him and never speak to him again. If he continues to harass you in any way, you will go to the police.

Or you can just skip to the blocking and end this charade.

If it were me, I would do option B. I think giving someone the opportunity to come clean and be real is best before just blowing them off and blocking them. I find with unstable people, this helps diffuse the blow up a lot. It will most likely still blow up in your face. He might get really nasty and try to shame you into backing down.

You need to get strong. You have every right to know who you are talking to. Don't let this person bully you.