Solution

get_avatar( $comment, 100 );

We are using that function to get the avatar, right? Well it supports a parameter for the default gravatar. So here is what we get.

get_avatar($comment, 100, "http://website.com/defaultGravatar.png")

Of course we can make it more dynamic, for example the image is gravatar.png inside of folder named images inside our current theme.

get_avatar($comment, 100, get_bloginfo('template_directory')."/images/gravatar.png")

So simple…

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