Most Helpful Girl
Dude, don't be a dick. Blackmailing her won't make her happy and will probably just make her want to leave you more. Delete the picture and let her go. She deserves to be happy and she clearly isn't with you. Trust me, if you let her go you can find some other girl who will most likely make you more happy. Neither of you are really satisfied in this relationship and honestly, it seems kinda toxic so I think it would be in your best interests to just let her go
Most Helpful Guy
Seriously man? The real question is why do you want to continue to be with someone who is threatening to leave you? I can only assume that she is threatening you with the breakup over something that you aren't really interested in giving her. If you give into that threat, then you just teach her that she can do that any time she wants something from you. Do you really want to be in that sort of relationship?
Now maybe she is making the threat for some valid reason. But still, if you have done something so bad that it merits her breaking up with you, then she should just do it and not string you along. Either way, that sort of person is typically poisonous and there are lot of others women out there for you to date.
As for the pictures, threatening her with them, or distributing them is a weak, dickish move, and you ultimately regret it if you become that person. Be the bigger person here and get rid of them.
Strong relationships are built on trust and communication, not secrets and threats.