I was going to answer this before but got side tracked. As far as I know there is no such plugin - at the same time, I think your reply is slightly ungrateful.
What would perhaps better illustrate your question is a screengrab? But, what to create such a plugin would require some work. Where you would store the signatures? How would the signatures link, by email, by username and so on? You then have to ask yourself, are you really going to force users to download images from say 15-20 comments each at 100-200k in size?
There might be something similar to gravatars that could work or perhaps you can make a request for such a plugin - I don't there is that much demand for it, simply because gravatars do that very job - stampling an identity to a user's post and giving it individuality. It's the modern "signature" for blogs, and I would recommend you take a look at it. There's even a plugin for WP that makes it VERY simple for users to register their gravatrs so when they make a comment, it displays their picture/signature/gravatar - whatever you want to call it with their comment.