What you want to do is not a rotation. As you said, it's a flip. For this, use the ippiMirror function.
Matthieu, For this partically case ( angle == 90) - yes, the Mirror function is the best choise, but for arbitrary cases - first of all call ippiAddRotateShift(,,,,,) for computing shift values of an images... . The function helps compute shift values xShift, yShift that should be passed to ippiRotate function for the rotation around (xCenter, yCenter) to take place. For your cases, it seems to me that shiftY == 2 ... --Gennady