Quote - Poser 6 because it has the shadowing function on the ground plane that allows you to create a shadow on any background image. 

That depends really.

If you are referring to using Daz's Cyclorama or RNDA's Infinity Cove, then you can get perspective shadows on the background.  But images have to be specifically mapped to each of those.

If you are importing a background into Poser, light and shadow won't interact with the background and you will have to add the shadows and light to the background in post work.

However, if you  use a primative single sided square and put the background on it inside the Material Room and then resize the square to the dimensions of the background, then the background becomes a prop. You can move it around and resize it and the light will interact with the background like it would any prop.  Your props and figures will also cast a shadow on the background, but only  to the degree that it would on a wall, but it won't be perspective.

Your other way is to render your image without a background.  Keep the ground plane visible so that your images cast floor shadows (you can determine the strength of the shadow by changing the shadow amount in the parameter dials for a light). Save the render as a .png file. Open the Render into a graphic program and then paste in a background of your choice and move it below the image.

"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