In my head, I think this is the easiest way for all parties ;
1) Report your original image. (The "Report" button under the image) Write that this is your own image and that you wish to replace it because of whatever reason.
2) Wait for a moderator to have a look at it. As soon as a moderator has marked your images as to be replaced, you can continue to the next step.
3) Upload your replacement with the "Replace" button that now have appeared under your original image.
To me this is the easiest way for everyone when you want to replace a image

Any thoughts?