I was just about to post on this same subject, I just noticed it today.   All background pictures get shrunk during Render.
I have a Macintosh G5 running OS 10.4.8 and Poser 6 SR3 and it has worked fine in the past.  But then again I dont know what version of 10.4.x I was running or what SR I had when it last worked.

Well, crap.  Looks like a reinstall is in the cards, as I have work to do.

I'll let you know what happens.