Hi prince, If you are just wanting to replace photos for one listing...you can replace them by going to the images tab then select manage images...when it loads find the photo you want to replace and check the box next to it. Then hit the replace button...you will now be able to upload a new photo. After you save it, it will reflect in your active listing because it will have the same url only be a different picture.
I think this option is only good for single listings with its own unique photo. If your running multiple auctions using the picture you replaced..its going to replace them in every auction thats using that picture, At least that would be the logical assumption anyway. I could be wrong, I have never done more than 1 listing photo change. Good luck