The quickest and easiest way to do this is to delete the 'wrong' picture, assuming you don't need it currently in another active listing, and re-upload the 'right' picture with the exact same filename as the 'wrong' one.
This way when you refresh the page anywhere the 'wrong' picture was showing up, the 'right' one will now instead.
Alternatively you could take the URL of the correct picture and edit your listing from eBay, find the wrong picture reference in the HTML code and swap out the URL's (which you would need to do if your using the 'wrong' picture in a different listing currently.)
