The images where obtained by taking photos of the messages on the screen of another phone without the permission and or knowledge of the person who owns the phone, the person is now threatening to expose a relationship by sending the pictures to another party unless I remain quite about another incident that has occurred
Then I don't think this falls within the scope of the offence of criminal blackmail under the Qld Criminal Code which is as follows:
416. Any person who, with intent to extort or gain any property or benefit or the performance of services from a person-
(a) accuses or threatens to accuse any person of committing any indictable offence, or offering or making any solicitation or threat to any person as an inducement to commit or permit the commission of any indictable offence; or
(b) threatens that any person shall be accused by any other person of any indictable offence or any such acts; or
(c) Knowing of the contents of the document, causes any person to receive any document containing any such accusation or threat as aforesaid is guilty of an offence.
Therefore, this would be a civil matter. Apart from alleging trespass to personal property, I suppose you could make a counter threat that if he exposes certain information about you, then you will sue him for defamation??? Although you would have to make sure you would meet the criteria to bring such an action - bearing in mind all the potential defences that may be available. There was a case where a guy published pictures of his ex-girlfriend nude to get back at her when they broke up, and she successfully claimed defamation. I don't know if this is the type of thing you have been threatened with though. Here is the link to a fact sheet: http://www.qpilch.org.au/resources/factsheets/Defamation.htm
Other than that I'm not sure what civil causes of action you have????? I suppose if he is revealing something about you that is true, but that you are trying to hide from someone else, then you may have to simply deal with the truth being known.