So the sky is at about -300 on the Z axis.

It's a moot point, but that doesn't actually tell us how far away it is. Minus 300 what?

You might try using the Ambient input* for your background image map instead of the PoserSurface diffuse lighting node. A background (particularly the sky) would not normally be lit by your scene lights - it would be self-lit. I'd hazard a guess that removing the diffuse lighting node would also eliminate those artefacts.

(* or the Alternate_Diffuse, as in seachnasaigh's example).

Edit: don't forget to set Diffuse_Value to zero, or the colour to black.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)