Move the mouse pointer over the image. Two buttons with arrow will appear at the bottom.

Select either Rotate the image 90 degrees to the left or Rotate the image 90 degrees to the right.

If you want to keep the picture rotated in this way, click Save.

そうでなければ、画像を保存しないでください。そうすれば、次に画像を開くと元の方向に戻っています。