It would be very useful to have the ability to rotate images on the horizontal axis, similar to the way the “Flip Transition” works.
YOU CAN DO THIS!! But I assume you mean “flip” when you say the “ability to rotate images on the horizontal axis.” Correct? Just right-click on the image to see the drop-down menu, then go to “Layer > Mirrored” – that flips the image left to right/right to left. Hope that helps 
I meant the ability to animate an image rotating horizontally like a 3D object, the same way that the “Flip Transition” works. The current rotation feature only rotates vertically on the axis. I’m talking about adding a “Z” axis in addition to the “X” and “Y” axis. With this feature it would be possible to scew images and animate them in a 3 dimensional space.
I thought that might be what you wanted, but you used both the words “rotation” and “flip” so I wasn’t sure, and went with the wrong interpretation. My mistake. You want to rotate in the Z-axis in “real-time” as an animation on a single slide (not as a transition). You could STILL do that, but only if you have ONE image that you want to rotate, by just using the Flip transition and adjusting the timing. If you have at least TWO images on a slide and want to animate just one of them, you can’t do it. As Oliver (at Boinx) stated, let them consider it. I for one would be glad to see ANY feature that offers greater independent control of different ways to present slides…