I don't have an answer either. Do a google search for something like "my pictures are sideways with xxx", with xxx being your camera model or whatever you are using for your pictures. Some cameras have a feature called something like auto-rotate. Try turning that on or off and see if it helps.
Only other thing I would suggest is trying rotating the picture in graphics software, even if it's just to rotate it and then rotate it back, then save it as a JPG. After you do that, then upload it and see if it is still sideways.