Rotating & Spinning
To rotate a frame, first, select it, then move the cursor over the rotate handle for that frame.
Press and hold the left mouse button. Drag the mouse. The frame will rotate as you move the mouse.
When you are satisfied, release the left mouse button. You can press the 'esc' key to abort the drag if necessary.

This control can also be accessed via the Frame Editor under the Position & Size options.

To spin the image WITHIN the frame, to correct for a crooked camera angle or for an effect, hold the Ctrl-key on the keyboard while moving your mouse on the rotate handle of the selected frame(s). This control is also accessible in the Image Editor under Angle.

Snap to 45-degree increments
While rotating your frame, hold down the Alt key to have the image snap to increments of 45 degrees (0, 45, 90, etc).

Snap to 90-degree increments
Images can be rotated via the Image toolbox. Click the Rotate option to open the Orient tool, then click on either of the two rotation arrows (items 1, 2) to rotate 90 degrees.

 
Snap to vertical
To snap a rotated frame back to a vertical orientation, select it, then press Ctrl-Page Up OR right-click on it and choose 'snap upright' in the resulting contextual menu.

Flipping
Images can be flipped left/right and top/bottom via the Image toolbox.
Click on a frame to select it, then, from the toolbox, select the rotate option to open the Orient tools to rotate or flip your images.
 

Tilting
Images can be rotated within their frame. You can use this feature to correct for cameras that were not perfectly level when the picture was taken or to create a fun effect.

Select the frame holding the image that needs to be corrected. Either hold the Control key on the keyboard while rotating the frame's rotation handle, OR click on the 'Image' part of the Frame Toolbox to launch the Image Editor, then change the value for the ANGLE parameter. You can press the 'esc' key to abort the drag if necessary.