If you would like to rotate your picture in any direction you first need to select it first by clicking on it.
Once selected you will have the editing menu appear above the album as shown below:
The option you are interested in is the orientation. Once you click on it the menu will change into the following one:
Now you can modify the orientation of the image you selected. Clicking on the Left/Right buttons will orient the image in a °90 angle. If you want a more subtle change you can use the bar to the left fo the left/right buttons.