After a lot more work than I expected, I think I've got the importer to work like it should. I've tried to make the image fields use the ubercart_images directory programmatically, but site administrators should check to make sure they are set to anyway.

I was trying to use boolean shortcuts to only download the files when they didn't exist, but I think they were being copied anyway. Or it might have been something CCK was doing. Whichever it is, I've gotten to the point where duplicate images don't happen anymore. Hopefully it'll be the same for you.