Dopers I’ve met, I picture when i see their name.
Others, if I’ve seen them on MySpace/frappr or wherever, I picture that.
The rest, I get some sort of variable mental picture based on what I’ve read, or know about them.
Similar interests, mostly. I glance through threads and don’t really take conscious notice of most names but if I wind up reading enough threads that that person has posted in, it starts to become more conspicuous, somehow, and I start remembering more about them. A good number of my favorite posters aren’t even ones that are all that well-known, at least if the various MPSIMS popularity contests are any indication.