The nickname came from Don Salieri from Mafia: The City of Lost Heaven. I think I was 10 at the time I finished it. Up to that point I always used some kind of a scramble from my name as my web nickname. I decided it sounds cool and used it to register my first Gmail account, and I (totally not by accident, yeah, totally on purpose) misspelt it making it Saileri. Since that day it never ever was taken anywhere I would register. So, if you ever met Saileri with this exact spelling, that was like 99.9% chance me.
As for the avatar, I once found out about Armoured Angels artbook and me loving girls with guns (or girls in butler suits, mhhhhhhhhhhhhm) just viewed it as perfect. Never switched my av ever. I think it was some CS:S fanart before that.