He then tried to upload his improved version of the
image.
[..]
Long-term solution: Replacing the image should be
transparent. He
should not have to care where it is hosted, it should just be replaced
wherever it is.
Apart from the valid points, I believe this is a fringe case. The case in
which you should upload over existing images are few. The alternative of
uploading with a new filename doesn't seem so counterintuitive to me that it
should deterr a lot of contributors. But I could be wrong here.