Zencart/Gallery2 integration: configuring for downloads

I am trying to set up the ZC/G2 integration to provide a system whereby photos can be uploaded to G2, and customers see only the thumbnails but can purchase full size images using ZC to checkout and handle paypal payments.

The idea of using the integration rather than simply selling photos using a standalone ZC setup is to provide a nice, simple interface both to the customers, and to the uploaders of the photos - the uploaders need only deal with the simple G2 upload facility, and don't have to touch, or even necessarily know of the existence of, the ZC admin section.

I have the same question as one which has been asked before and given an inadequate answer.

I'm sorry, but those pages do not give enough information to set up the Gallery2 integration.

In particular, the second one talks in terms of "files uploaded to the download folder". That obviously doesn't apply to the ZC/G2 integration case. ZC ought to be picking up the location of the file uploaded to G2 automatically. It isn't, and nowhere can I find any elucidation as to why not.

If I work around this by entering a filename by hand, the product does then appear in the downloads manager with the green dot next to the filename. However, I can't make it do this automatically. Every filename has to be entered by hand for every uploaded item. Obviously I am missing some step; please tell me what it is.