Actually I think that any tag that gets input that is unexpected ought to display an error message. For instance the URL tag if it gets a badly formed URL. Most posts don't get a preview before they're submitted, so on that page that pops up after submission would be a good place to post the message. Something like "Some problems were found in your post" - then have a list of what issues were found like "the image tag found content that it was unable to parse" or "the image tag found something that was not an URL"
I agree that this would be a feature used by a minority of users, my first thought is that the only place I've seen it in use is with emails. Leaving this to be implemented as a MOD is a good idea. I might make a suggestion for that MOD, convert the image from the text representation and save it as an actual file on the server. If you're going to server the image anyway, let Apache take care of the static content and don't munge up what's in the database with several thousand bytes of MIME text.
I might also suggest for that MOD that any image on remote servers be (optionally) copied locally - it would solve that problem previously noted about remote server issues and files that get removed.