Hi,
Unfortunately, as the default avatar is served by Cravatar, it will always show users without a Minecraft account as Steve as this is something Cravatar does. I might be able to add something to change this in version 2.
As for changing the default picture of someone who does have a Minecraft account, you can do so by changing lines
392-407 of
core/classes/User.php to point to a path/URL with the new default avatar.