I think I know a fix (if memory serves me correct...)

At least in Poser 7, and I THINK it was the same in Poser 6, when you open the material Room and click "Background" the BG colour node is attached, and has a grey colour. Now, using the "use as background picture" plugs the BG Picture node in instead, but rather than this, try setting the BG Colour to white and then plug in a texture map there, with your background image.

It's a wellknown Poser bug, this. And IIRC, this is the easiest way to fix it.

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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.