I fixed this a while ago on my PC. If I remember correctly I downloaded the image, rotated it in a picture editing software, re-saved it as a new image, and re-uploaded it.
I don’t know what causes it to flip in the first place, though I have a sneaking suspicion it has to do with phone orientation (do you rotate your phone left or right?) and the camera app misbehaving with the 3DHubs website. I have not confirmed this nor tested it, but if anyone else has let me know.