you might say "flipped horizontally." That term is also common in computer graphics applications.
A computer image and a mirror image are true two-dimensional "objects" and I doubt that early speakers developing the language really considered that possibility or gave much importance to it. For a three-dimensional object, I think this transformation will always result in some other change which the language would seem to consider more important. If you horizontally flip a computer image, you are still looking at a computer image which someone may not even be able to tell has been flipped (unless there's writing or some other clue). If you print that image on paper and then horizontally flip it, you are looking at the blank side of the paper. It is much more important that you have turned it back-to-front (wrong-way-round) than that the left and right sides have been exchanged.I was looking at myself in the mirror in an elevator, and I asked myself how to say the image was left-right reversed.