I told him it couldn't be done in iPhoto. If you find that irrelevant, you shouldn't have said the same.
Pixelmator'll do it, as would DoubleTake or PTGUI.
The way you did it without useful alternatives made it a useless response to the O.P. and doesn't get him anywhere.
I gave him a complete answer that gets him further. And I added the fact that iPhoto can't do it as a reason to try the alternatives I mentioned.