Quoting fuel:
I would say it's your phone at fault.. the original photo, even though the phone/camera was pointed in 'portrait mode' while taking the photo still would have actually snapped the photo in 'landscape' orientation and has the information in the image meta data that it needs to be rotated x amount of degrees to be viewed. Your phone gallery is doing just that, as does Windows explorer etc. The forum doesn't, and it shouldn't need to. If you take the photos in landscape mode to begin with you wouldn't have the issue.
I think you are right but it is frustrating that most computers (PC or Mac) clearly have the ability to distinguish which way a photo should be displayed but don't communicate this info when uploading. I presume the meta data file includes info from gyro sensors or half the portrait photos I take would be upside down.
I recently downloaded pictures from my camera to the desktop prior to uploading to the website. There were a selection of both landscape and portrait shots and they displayed in the correct orientation on the screen. When I uploaded them however every portrait shot was rotated 90 degrees. I went back to the desktop rotated the pictures 90 degrees, saved them and uploaded them again. No dice, still off 90 degrees! Go back, rotate to the correct orientation on the desktop save the picture again and upload and everything is hunk dorey.
Essentially therefore you need to manual rotate photos to the correct orientation and then save them as a new file in order to 'tell' the PC that yes you do actually want them displayed like that, even though it seems to understand that in the first place.