Maybe I'm missing something, but I can't figure this on my own.  I want to render a model with a transparent background, but I have imported an image as a background. 

I know that I can turn the GROUND prop visibility off and set Superfly to render a Transparent Background, and the result is a .PNG with a transparent alpha layer in Photoshop: works perfect if there is no background image.

For some of my models, I've been scanning in my own artwork to use as a reference.  I put that image in the background and then I can line up the Poser model to make it match.  However, once I import a background image, I can't find a way to "unimport" it to be rid of it.  I've tried replacing the image with a transparent .PNG, but that isn't working.  My current workaround is to import a blank grey background, which is fairly easy to eliminate in Photoshop.  It would be more elegant to not have to do that, though.  The background isn't a prop, so it can't be made transparent... right?  Is there an easy fix for this?