Quote - Go into the material room for the square. Load your image into an Image_Map and plug that into Alternate_Diffuse of the Poser Surface root node. Turn off all other material effects, i.e. set Specular_Value = 0 and Diffuse_Value = 0.

We use this material arrangement so that the prop does not get lit or receive shadows. We don't want to light the backround photo.

I've always connected the image_map to the Diffuse Colour with a value Diffuse Value of 1. I turn Specular Value to zero. 

Then I set up the rest of my scene according to the background image.  Then I hide the background. Render the figures and props with cast shadows.  Then hide the figures and props and then render the background on the square. I composite them together in Paint Shop Pro.  Because I had the scene already set up the way I wanted it, the background and figures .pngs match up perfectly.  Trying to get your scene lighting to match that of a ready made background is futile.  I think it's better to have your background interact with the lights in your scene.   This way the image takes on the lighting from the lights in the scene which IMHO gives a better result and makes the background actually look like the figure belongs with it because of the similar lighting.

I just tried the method you suggested, but I don't see the image in the Pose room until I render so it's impossible to gauge where to put your figure and props in relation to the background picture. Also, wouldn't connecting it to the "Alternate Diffuse" node prevent it from interacting with the lighting? 

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi