The biggest problem with doing it like that is there would be no way to restrict size in height width or filesize when doing it that way. Someone could set themselves up with an avatar that is 2000 pixels wide and 1 MB in size with no trouble at all and tear your threads to shreds. :speechless:
And people that didn't choose an avatar via your method would suffer red X syndrome in IE in their posts where the avatar is unless you set the field with a default value via a DB query.