The difficulty with photos at 90 degrees from what you expect starts from the moment you take your photo
If you take your photo in portrait mode, so as to accommodate a tall object for instance, your subsequent review of the picture on the camera will detect this and show it in portrait mode so you see a tall thin version
Similarly the same photo uploaded to your pc will automatically rotate the picture so it looks right, but in reality the photo is on its side and your pc is just being clever too
Unfortunately the forum software does not recognise the orientation information contained within the photo
To fix this, on your pc....
If you rotate the photo using the rotate button and then save the the picture is on its side both in reality and as you see it
Rotate it back and save it and now it is in the correct orientation in reality and as you view it
You need to force a save by moving on to the next photo and then back again
The rotate and subsequent save action are required to make this work in each of the two steps above
This resets some of the orientation data inside the phoot so that when you upload the picture to the forum it will display the photo as you would expect
Of course the alternative option is to resist the temptation to rotate your camera in the first place......
Certainly, by comparison to a few years ago, the resolution of the photo will be equaly as good without turning your camera