Hi assuming the image is one included when the listing was created on Auctiva you have two options dependent on how comfortable you feel about editing HTML.
1) Tweak - in ebay -> revise item identify the arrangement of the code for the images and just delete the appropriate section.
2) Complete replace - modify the saved listing to delete the surplus image and save it.
From the saved folder select HTML for that listing and then copy/paste all the HTML code for that listing directly into the HTML description window of ebay->revise item to replace all the existing code.
The second is the easiest believe it or not.
In either case preview before submitting/committing the changes to ensure nothing has been broken.