As stills render correctly with your background image, then it sounds like this could be a Poser bug.

Are you rendering your animation to seperate images or are you rendering the whole thing direct to an AVI file ?

If your doing the latter, you should try rendering to seperate images and assembling them afterwards.
Obviously if you're already doing that, then it maybe that there's a subtle difference between your render settings for stills and those you're using for the animation.