Hello again -
there is an option available in your image management area where you can replace an image - however, this will replace the image anywhere it is used.
If you are in the Image Management area of your account you can see that each image has a unique URL and the Replace tool that is just above the table that shows your images will allow you to swap an image for a different image and the new image will use the URL of the originally uploaded image. This changes what shows where that URL is used, so it will change the image that displays on your active listings.
To use this, locate the image you wish to replace in your image management area, check the box to the left of the thumbnail and then click the Replace button. This will open the tool along the top of the area where the images display where you can browse for your image to replace the initial image with.
Alternately, you would need to update the saved listing with your differing images and use the HTML from the saved listing to update the listing on eBay using the Revise Your Item form and the description editor in HTML mode.
If you have further questions about this or run into any issues, please feel free to file a support case from under the Help tab of the site and we'll be happy to assist you further.