As other have said, its because you have taken the picture with your phone in landscape mode. When you post, it loses that "it's in landscape" tag and appears rotated. Try posting a picture taken in portrait alignment - I bet it won't be rotated. If I'm correct, sadly you will need to edit and save the picture on your phone rotated for those you wish to post. I had this happen to me a long while ago (iOS) and had to do the steps outlined. It was a real PITA! In my case, an iOS update fixed "something" and I no longer need to go through that faff.