Heh, sorry about the short answer. Was in a hurry

There are basically too methods of using reference images. One is with the built-in image planes and the other with some real geometry like polyplanes with the reference images as textures. I wrote a little MEL script a while back for the latter approach.
You can find it in this thread: https://forum.simplymaya.com/showthre...&threadid=2032

If I remember correctly the video tutorial I mentioned above deals with the built-in image planes...