I get what Pandan means.
Some users, and often times those who have the same avatar for a very long time, have avatars which represent their character (atleast to some extent) on the forums.
And then there are those who are just noticable through their avatar due to having used it for a long time.
I've been told that my avatar is recognisable.
I don't know how it would represent my character on the forums, if anyone does find it to, but none the less, I don't doubt that people can look at my avatar before my user-name and know who I am.
I don't really associate many aspects of avatars to different things however, like cooldawson suggests.
Maybe a low quality image, or a single-frame of an obviously meant to be animated picture might tell me that this particular user isn't very good at working with images or forums, but that generally is where it stops.
After that, it's just a few users who happen to have representative avatars of themselves, and then it's just associating users with their avatars.
I will admit however, that I've often found myself very confused at who certain members are, particularly those who are big fans of, or are frequent users of the My Little Pony topic.
And I think that's simply because a lot of them use MLP avatars, and those avatars consist of the same characters again and again.
So, I can never really differentiate between users. Atleast, not off the bat like I can with other people.
But, the same is true whenever a fad jumps up, or a group of members take on avatars of any other sort of fandom.