Overlay / Alignment...
After you have used 'Add Overlay Image', you may use the following keys to adjust the background and move it in 'stereo space' :-
Alt+Up
Alt+Down
Alt+Left
Alt+Right
The overlay image is always located at the stereo window and is not affected by the above or normal arrow keys.
On some systems, the above keys may not produce a sufficiently-fast response and an alternative method is described in the 'Add Overlay Image' section.