This is one of my biggest pains in the arse. Different sites where I want to post pics display the pic differently. Sometimes the destination site will give you an edit feature that will allow you to rotate it, other times you have to figure out how to rotate it on your phone.
Yes, the answer is "by editing it on your phone", but unfortunately I can't help much beyond that. I've always been in the android clan and have never owned an apple product, so I can't offer any specific direction there. Hopefully someone will be along shortly that can do so.