I usually post pictures from my iPhone and I just go into edit on the picture and turn them right round in a circle and save.
It usually looks exactly the same as when you started, but as you are viewing them with the volume button facing up, the phone twigs that that is the right way up and posts them they way you want.
It will show you the picture on the phone the right way up whatever way you hold it, so that is no clue to the actual orientation the phone has it on.
Makes sense
Thought not, but it works for me