Opto has the right idea.
2 images of the same image, one reversed, or it actually be anything the same size (just for visual smoothness).
Key the first image to shrink bringing the left and right sides together till they all but disappear and then, after previously keying the second image to near-nothing expand the left and right sides to the size you intend.
It will appear to be rotating.
I've used transitions, also.
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