This is a shot in the dark but it helped me in a past project, and it is referenced from page 90 "Maya 6 - The Complete Reference" text. The problem might come from you adjusting the perspective view of your project before setting up your image planes. So try:
Starting a new project, choose Window | Saved Layouts | Four View, or click the Four View button in the toolbar. This will bring up the four panel views, with one perspective and three Orthographic views. BE CAREFUL NOT TO MOVE, PAN, OR SCALE IN ANY OF THESE VIEWS! The image planes will be positioned and scaled to frame themselves within the visible region of the view window. Zooming out, or panning in one window will cause the image plane to be larger, or skewed in that view while the other image planes will remain at their default and will result in your underlay images being out of proportion. If you have already moved around in one of the views, you'll need to start a new project.
Could be your problem, but also might be worth a quick try... Good luck, nice illustration too!
