It is an EXIF issue. (Exchangeable Image File Format) Older browser supported forums, websites don't use EXIF data controls, or if you are using an older version of an iPhone or Android it doesn't support EXIF parameters. This version of vBulletin most likely doesn't use EXIF.
A newer forum format will as well as have auto-resizing capabilities built in for downloading pics.
If you'll save your photo to a download site such as Flickr or Photobucket first, then upload it here you shouldn't have that issue.