Ok, well reading thru all these forum posts, my head is spinning! Geez I feel stupid, but I just want to make sure I have this concept right. Please correct me if I'm wrong, but as I understand it, here are the steps I take:

1. Open a Poser scene with whatever characters and props I want.
2. Select a background picture. Insert a one-sided square into my scene, and set it's X, Y scales to match that of the width, height of the background image. Create a new image map node and attach it to the alternate_diffuse node (diffuse value=0, specular value=0)
3. Rotate and move the one-sided square prop to where I want it. Resize it if I desire, using the "scale" dial so it maintains aspect ratio
   This is where the whole thing gets confusing for me...
4. Insert a hi-res square prop to act as the "ground" for the scene.
5. Using different settings in the material room, as well as lighting, this prop will  be transparent, except for the shadow. And if I've got the settings right, the shadow will appear to be part of the background picture, making the characters/props look as if they're actually in it.

Have I got it right? Originally, I was thinking that the shadow catcher was some kind of miracle prop, but in reality, it's just a series of settings in the materials room, right?
I've been reading thru all the different posts about it, and I think my skull's going to pop! But I want to get this right. If so, I feel like it'll open a whole new world of possibilities for me. If I'm completely out to lunch on any of this, please let me know. Thanks -

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