Yes. I think the easiest way is not to use the Poser Background at all, but use a prop (e.g. one-sided square) with the image attached. Adjust the dimensions of the prop to suit those of your background image and the render dimensions.

If you plug the image node into the Alternate_Diffuse channel on the PoserSurface, it will be self-lit and will not react to any scene lighting. You can adjust its brightness using a greyscale shade in the Alternate_Diffuse colour.

To see the image in Preview, plug the image node also into the Diffuse_Colour channel, but set the Diffuse_Value to zero.

(edit) there are no scene lights in the above render - both the square and my environment-sphere gradient shader behind it are self-lit.

"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)