Of course, you must be logged in to your facebook account.
Next, click on the album where your photos are located.
In the album, click on one of the photos that you want to rotate.
Below your enlarged image, you will see the icons for facebook's image rotation tools. Just look for the arrows that point to left or right.
Click on the left pointing arrow if you want to rotate your image 90° to the left. The right arrow will magically rotate your photo 90° to the right.