Open it in the windows photo viewer. Rotate it left (or right, doesn't matter). Now click to the next picture (or just exit), this will save it in that orientation. Now reopen it, rotate it right (or back to upright), and click to the next picture (again, you can just exit, just make sure it saves). Now re-upload, should be fine.
Reason this happens, is the camera takes the picture however it's held (if it's upside down, the picture is taken upside down). But newer cameras have sensors that determine how the camera's held, and put information in the picture to indicate such (i.e., hey I was being held upside down, so totally flip this picture upside down for it to look right). So your computer automatically shows it the right way. But the picture itself is still upside down. Most (all?) forum software is not sophisticated enough to read this orientation tag, so it just displays the picture as-is. Changing the orientation in the Windows Photo Viewer (and then exiting the picture; rotating it left then right without exiting the picture, will result in no changes being written to the file) changes the actual picture itself.
Happens to me all the time. Picture looks right on the computer, I upload, it's sideways.
