Draug, I've seen smart theologians argue both ways on that one, and in truth, I don't think the arguments either way are sufficient to make an iron-clad case. The most you can say is that the case is not strong enough on either side to draw firm theological conclusions. (Yes, I looked at your article. Parts of the argument were strong, parts were weak. The weakest, in my opinion, was on the cohortative mood in the analysis of "Let us make." The plural refers to whoever God is speaking to? Really? So there are other beings helping God with creation? And man is made in the image of all of them? While not impossible by any means, that would certainly be in tension with the rest of the passage and the rest of the OT, at least as much as trinitarianism.)
I'm not saying this argument can't be right. I'm just saying it's not clearly right, and there are smart people on both sides of that issue. (If you have access to online journals, see e.g. "The Imago Dei and Election: Reading Genesis 1:26–28 and Old Testament Scholarship with Karl Barth," by Nathan McDonald, in the International Journal of Systematic Theology, Volume 10, Issue 3, pp. 314 ff).
As for interpreting OT prophetic passages, it's true of course that you can't go to them already with the explicit premise that they refer to Christ, but there is nothing wrong with arguing that they do, based on what they say, the arc of redemptive history, etc.