When you take a photo Portrait, that photo is in the phone as Landscape (the wide view)
When you post, if the forum reads the data, you have a tall photo = Portrait alignment but sideways.
You can open in any editing software and rotate the image. Problem is sometimes that software will read the EXIF data and correct, without changing to Portrait. Once you figure out your software, you will always have the same situation and know if you need to open > rotate > save or just open and save.
Or as an alternate, turn the phone sideways when you take the photo and it will be what you see.
To make things worse, some phones (iPhones are supposed to do that) will adjust the way the image is saved and some others don't.
You'll have to test and figure out what you phone does, then no more problems?