Hmm, good question. It's been a long time since I've installed the default avatar... It should be an installed plugin if I remember correctly.
Code:
if (!$this->post['avatarurl'])
{
$this->post['avatarurl'] = "customavatars/SLoverDefault.png";
}
if ($this->registry->userinfo['showavatars'])
{
$show['avatar'] = true;
}
This is what I have in my Plugin Manager for default avatars - Product : vBulletin, Hook location : postbit_display_complete
~ Domokun