That's exactly the problem I am describing. They are making their own portraits, but they aren't sticking to their usernames, at least for me. A visual icon would help. It could be small, 25x25 as someone suggested.
I'm not sold on the idea of something like this here, but even a very small icon (like 16x16 or smaller--very small) or bullet of color would would make threads easier to keep track of.
"Look at that 3 pixel noob..."
You could use the user's top bar preferences!
(I recently joined the elite - that's why I'm smug.)