Hi karensvariety,
Unfortunately, I do not know of a way to do this.
In order to use a gallery image when listing through Auctiva, at least that image must be placed in a "Select an Image" box on the lister page. Also, the "extra" image at the bottom cannot be deleted because it is the one image that is submitted for eBay's image hosting, so it can be designated as the gallery image.
-Mike