There already is a default avatar. It's shown if a user does not upload one. View your own profile
here
and you'll see the default given you do not have an avatar uploaded. You can of course replace this default avatar at the following location.
components/com_comprofiler/plugin/templates/default/images/avatar/nophoto_n.png
It is dependent on what CB template you are using, please keep this in mind when replacing the image.
As for setting it to display on registration simply edit the field in field management and configure it to display on registration. If you want to force users to provide an image then set it to required.