[kpmedia]

You'd have to a write/edit a lot of code. Furthermore, you'd want to permanently cache the results.

The biggest issues with something like this is the server processing overhead, and the slowdown of your site due to slowness connecting to other sites. You could probably start out with leveraging some code out of the "other server image caching" plugins (I forget the exact name of it). Add in some GD image processing. And then finally edit the bbcode that wraps links.

I could probably do something like this, but I have no idea how long it would take. (No, sorry, not offering, too busy. I just wanted to confirm that something like this is surely possible, and by using already-existent code.)

It would be pretty cool, sure.
But it could also clutter up a forum and wonk out your post spacing. Therefore, not cool.

You'd want to apply it conservatively, maybe only in certain forums by certain user groups. But if that's the case, why not just manually create images on such a limited basis?

If you know PHP, this isn't impossible.