You did notice that I said
558 copies of the same image in a total of
371 posts.
Yes it would be perfectly reasonable to see several instances of an image. It can happen when a post is quoted for example or a person might even refer to the same image in two or three different posts. When it gets to the hundreds it cannot be due to random selection by users, it can only be a bug.
If one has left the replacement image value blank as I have done, what should we expect to see? We have lots of entries in the replacement image of
http://mysite/ with no file reference in the URL. I have been reading those as meaning the image was missing from the source site.