Most Avatars would reflect that persons personality, or at least part of, but occationally you get someone who just randomly pics one that has nothing to do with them. It happens, but not always.
My current Avatar, the Terminator skull, I chose because it was the only one that I could find that I liked the most. I was looking for a few other possibilities, maybe Jango from Burst Angel, or a GUNDAM, Or something else with big guns . Once I get my post count high enough, I'm going to design a sketch, my sister's going to colour it (she's doing that kinda stuff at school), & I'm going to get it turned into a sig & avatar. I already have the character in mind (someone I created for the Naruto world), a idea of what it going to look like & a "script" for it. And since I based this character around a part of my personality (it's not me, he's just based on one layer of my personality), I guess that'll show of a part of myself too. Eh, whatever, just as long as I like the look of it.