Dear Sir, in that case, it will always depend on the terms of agreement between me and the advertisement agent. The Copyright Act is silent as to who shall be the copyright owner. This right can be transfered as any other property.
However, in such case, normally the company for which the advertisement agent took the photo has the copyright and only it can take action for the infringment of the copyright. I (or the model) does not have any remedy like defamation. If the advertising agent has violated the terms of the contract betwen the model and the agent, then the model can sue for breach of contract, but not for defamation.
I genuinely feel that in the matters of copyright, the question of defamation does not arise in any case.