Aqizzar and Jackrabbit are the only ones that have a stand-out voice. I always imagined Aqizzar talking like a newscaster, and Jackrabbit is Max.
I usually see most people as their avatars too, except in cases of real-life celebrities, and a few other exceptions.
Frumple was always a mystery, because I don't know who's in the avatar. It's just an indistinguishable silhouette for me.
Megaman never had an image in my head, dunno why. I guess my head is saying, "He's not really Megaman."
ToonyMan changes frequently also, so I have the tendency to default to a generic 90's shonen hero.
Nenjin's avatar always looked a bit terrible on my computer, so it's just some silhouette.
Ponies are a grey area, Sowelu and whoever has hipster Rarity are the only ones that pop into my head as being the same image in my head.