Also see: http://www.everyday3d.com/blog/index.php/2010/03/15/3-ways-to-draw-3d-lines-in-unity3d/

If you are using Unity, there are various ways to achieve this; one way would be to draw the lines connecting the 4 corners of the quad, using the GL draw functions:

http://docs.unity3d.com/Documentation/ScriptReference/GL.html

The code for drawing couldbe triggered from within your own implementation of the  ITrackableEventHandler, which allows you detecting when a target is found or lost, and which  should be attached to the Image target (see also the DefaultTrackableEventHandler script, as an eample, attached to each Image Target)