Hi bbqkaren,
In order to select the gallery option when listing through Auctiva, you just need to select your gallery image from the "Select an Image" box labeled "Gallery", as well as check the box next to "Gallery" in the "Marketing Tools" section of the listing.
If you configured your listing properly to utilize the gallery feature, and your gallery image did not show up once the listing was posted to eBay, please file a support request using the appropriate link on our help page:
http://www.auctiva.com/free-auction-software-help/help.aspx-Mike