Do you have any recording of them threatening you that they will leak the picture or anything like that, then you can approach the police with a formal complaint and register an FIR against them under section 506 of IPC - criminal intimidation.
For further queries or information, you can contact me on 7601090381 or firstname.lastname@example.org
taking nude picture without consent is itself an offense so do not get scared of her threat she will get into more problem if she try to make that picture public. secondly no one can force you to marry anyone without your consent not even if you have promised someone to marry. so do not lose your ground and stay firm in your decision is not marrying her. Get married with someone of your choice as soon as possible. Finally keep in touch with me if you want to take legal advice on every step that she takes against you. If i was in AP it would have been much easier because I could have helped you with your case in the court. nevertheless if you want legal advice then i am available at 9013429126.