You can rotate the image by clicking on the the three buttons or setting the rotation value in the number field. Click the left button to rotate the image 90 degrees counter-clockwise.
Click the center button to rotate the image 180 degrees.
Click the right button to rotate the image 90 degrees clockwise.
The Rotation (degrees) number field can be used to rotate the image by any arbitrary angle.
Check the Crop corners checkbox to remove excess blank space.
For a single image, you can rotate by clicking and dragging the image, use the Rotate layer.