There is no "up" sensor on the forum. While your smart phone may always show it in the correct orientation (knowing what position your phone was in when you took the pic) the forum will show it in the orientation your PHONE WAS and not your picture. Much like if you take a video with your smart phone sideways: playback on your TV will be sideways (meaning the video is displayed with a 90 rotation) despite showing correctly on your phone.
If your picture loads sideways you can save it again in the correct orientation and reload it (does not always work on older phones though).