Image redirects only works when there is no file at the redirected image name. So when redirecting an existing image, it has to be deleted first. In some cases it is useful to let non-admins see the file history of a redirected image. That is not possible when the file history is deleted.
Therefor it would be nice if "#REDIRECT[[Image:someimage]]" on image pages for an existing images would redirect the image, the same as on image pages for non-existing images.
Currently the 'upload' permission is necessary and sufficient to add "#REDIRECT[[Image:someimage]]" to image pages, even for existing images where it does not work. 'reupload' (or a new 'redirectimage') should probably be required to redirect existing images.