Hi skinny,
We actually introduced some functionality similar to what you have described within the last year. When you on the "Manage Images" page within your Auctiva account, the "Listings" column indicates the number of active listings each image is currently being used in. Therefore, if you are looking to delete the images that aren't being used in any active listings, you can safely delete all of the images that reflect "0" in the "Listings" column.
Any additional questions or concerns? Please file a support request using the appropriate link on our help page: http://www.auctiva.com/free-auction-software-help/help.aspx
-Mike