I tend to think of people more in terms of their voices. This is influenced by their aesthetics and posts I've seen from them, but I don't tend to have an 'image' in my head of people online, on PC or otherwise. For instance, Sam, I normally think of you having this gentle, proper, very feminine voice, rather than what you actually look like (though I've seen pics of you in the Selfie thread). Probably from the way you write and the avatars of the pretty, proper ladies you normally have.
If I don't really know the person, they have a 'neutral' male or female voice. I think I do it this way due to writing so much, where I have a lot of different voices in my head to differentiate characters based on their characterization. Just kinda comes naturally.