Maybe, but how would we do that in a minimal way that would still be self-explanatory?

The alt-text/hover text of the timestamp would be the perfect place for it. That way it wouldn't bias anyone who doesn't go looking for it explicitly.

But then the vast majority of users will never know about it, which is not self-explanatory.

Also, out of curiosity, how does that work on mobile? I know it's a naive question but I've never figured that out.

If you stick a title attribute on an image, it will show up when you long press on it in Mobile Safari. This doesn't work with links, though, because it'll show the destination instead. Might I suggest just putting the submission time in parentheses, like "1 day ago (resubmitted 3 hours ago)"?