The forum software doesn't observe all the metadata in images correctly and as a consequence, photos that look fine elsewhere end up getting rotated on the forum. It happens most to me with photos from my iPhone. What I have to do is open the photos on the Mac or PC, edit *something* on them (usually just add a small black dot in a black area) and save them. That process, for whatever reason, appears to update the orientation information in a way that the forum software understands.
It's been a problem for a long time and Shane did at one point implement a "fix" on the forum software, but it broke other things and was removed. I've just been living with it since.
Importantly, it's not you, it's everyone!