Did you actually post it and it was sideways or did it just look sideways while you were submitting? I’ve had pictures appear sideways but turn round when posting is complete.

You should take a photo side ways - in landscape mode. Otherwise you can rotate with picture editing programs on your PC.

There's a possibility that changing the photo size affects the orientation. The last time I included pics in my initial post, I selected "medium" when prompted and those pics, which were not sideways when I selected them, posted sideways. When I posted additional pictures in the same discussion I either ignored the prompt or selected "original size" (sorry I don't recall precisely what I did) and the photos didn't reorient. Seems worth a try. That said, I have also had the same experience that floral_uk described, though not recently.