Probably the easiest way would be to alter the defaults for a few values in the w3t_Users table. I'd suggest doing a database backup before doing this:
ALTER TABLE w3t_Users
CHANGE U_Picture U_Picture varchar(150) not null default '#url_to_image#',
CHANGE U_PicWidth U_PicWidth int(4) not null default '#width#',
CHANGE U_PicHeight U_PicHeight int(4) not null default '#height#'
You'd need to replace #url_to_image# with the full url to the image. Also change #width# and #height# to the actual number of pixels of the image. This should make it so anyone without a pic right now, gets one, and any new members will get it as well.
Like I said, make sure you do a backup first, at least of the w3t_Users table that is getting altered.
