Some users I see commenting often in communities I read I associate fairly closely with their userpics.
When they change their username, I'll think of the old one whenever I see their icon (e.g. madcaptenor will probably always be "izzycat" to me, at least as long as the default userpic stays the same), and if they change their default userpic, I'll be a tad sad whenever I see them post but not with "their" pic.
...which makes me wonder whether people are like that with me, too. I can certainly imagine that there are people who associate "pne" with my Martin userpic, since that was the first userpic I ever uploaded, it's been my default nearly all the time, and I post (at least comments) with other userpics comparatively rarely.