Why not just use the Poser ground plane, in P6 it's default set up to have shadow cast on as its only feature, it's not even visible but it catches shadows... So the moment you load a background picture you will get the shadows... Or am I missing something here?

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.