I tried Poser 6 with a background image and I got an alpha channel around the subject in the front of the background image. Didn't know that could be done.  In Poser 7 I opened the same file and I get an alpha channel that is a solid block the size of the image which I believe if the same behavior of earlier versions of Poser, except version 6. I don't normally use background images because I couldn't get an object out of the image with the alpha channel, so I never even tried that in Poser 6. I'm saving in psd format which is my usual.