I haven't voted yet, because I'm not quite sure what I do. I seem to develop vague mental pictures, but no more than that. I remember when I met Wendy, I was very surprised because she didn't look much like the picture I'd had of her. At the same time, though, I couldn't say why because the picture was so vague there was nothing I could point to and say "I thought you had..." (Wendy sticks out in my mind because she's the first FoLC I met in person before seeing a picture.)
I've been lucky in that I've been around long enough and met enough FoLCs that I've at least seen pictures of a good number of them. So that helps. Otherwise, I just sort of gather information in bits and pieces ("okay, she's about that old, and she says she..."). Sometimes, I'll fill in a few more details based on personality. Those will often be wrong, but I never really expect them to be right. It's weird, I know.
To answer the questions so far... I'm 26, I used to have brown hair, but lost it in my early teens, I do wear glasses (avaitor style, but thin frames. My latest pair is made of nigh-indestructable flexible titanium, with hardened plexiglass lenses), and I'm about average height (5 foot 7 or so).
I do own a beanie copter hat like the one seen in my avatar, but I almost never wear it. Nor do I generally wear a mask. I am also several years older than Tim Drake (the Robin whose picture I "borrowed" to create my avatar). I did wear both the hat and a mask to NEFF's costume party, so at least one picture exists of me as "Hatman: Avenger of Boredum (and bad speller...)"
Anna... Uhm, yeah. Really extraodinarily good looking. Kind of like a cross between Dean Cain and Jude Law, but with Michael Jordan's suave baldness. Sure, why not? (Okay, in reality... I'm the best-looking guy in this room. I know this because it's my room and if there's anyone else here, he's invisible. Invisible people don't count as good-looking because they're invisible.)
BTW, Vicki, if you want help resizing the pic, just ask.
It's not that hard to do with any half-decent image editing software.
Paul