I am making some tests now to verify this, but I see some IBL experts online now so I thought I'd alert you now.  I've recently made a couple of dozen IBL lights and corresponding panoramic sphere maps.  As a test I made a compass image, just colors and letters for cardinal directions and assembled a LatLong and Probe in the same way I made the scenes in Terragen. 

It appears that the light object Poser is using does not properly translate the directional images.  That is, East shines light from the East, West shines light from the West, Up shines light from Up, and Down shines light from Down.  However, North shines light from the South and South shines light from the North.  

The light is inside out.