Honestly I don't know about the signature. I don't think we allow outside-sources, and it shrinks it if you upload it to the site. Maybe someone else who's used a picture sig can weigh in.
As for the avatar, send me the original size image and I'll resize it and upload it for you. For future reference, we've got an avatar-request thread in Editors Pick that I'll go find in a minute.
EDIT: Future ref:
http://www.musicbanter.com/editors-p...-requests.html