Custom Comment Gravatar

When someone made a comment without Gravatar email association, a default image is shown from Gravatar.

How do I change that to use a different image?

Comment.html

<img src="//www.gravatar.com/avatar/<perch:comments id="commentEmail" type="email" label="Email" order="2" hash="md5" required="true" />?s=120&d=mm" width="60" height="60" />