On the topic at hand, I agree with DocMoriartty that the cost of litigating against Apple could be prohibitively expensive, even if the "Imag" party were correct. On the issue of whether the Imag trademark would infringe the iMac trademark, it depends on too many factors to be definitively stated.
My advice would be to talk with a good trademark lawyer before you start using your mark. It will be substantially less expensive to be proactive and take steps to ensure that your mark does not infringe, than it would be to just starting using that mark and then end up in a lawsuit. This is not to say that receiving advice in advance is guaranteed to avoid a lawsuit, but it will likely reduce the risk of a suit substantially.
My .02
My advice would be to talk with a good trademark lawyer before you start using your mark. It will be substantially less expensive to be proactive and take steps to ensure that your mark does not infringe, than it would be to just starting using that mark and then end up in a lawsuit. This is not to say that receiving advice in advance is guaranteed to avoid a lawsuit, but it will likely reduce the risk of a suit substantially.
My .02