Hello gibsonbk -
If you need to change out an image on an already active listing you can do using the Replace function on the Image Management page. You'll first need to locate the image in question in your Image Management area and select the checkbox to the left of it and then click on the Replace button (next to the Delete button).
The page will then prompt you to browse for another image and once you select it and make the change the tool swaps out your prior image for the new one using the URL of the initial image, so that change then shows on eBay.
If you encounter any difficulty with this process, please feel free to file a support case from under the Help tab of the site and we'll be happy to assist you.