Searching in google I have found this article very interesting:
where the author expains very well the simple steps to do to set your own default avatar.
To change the avatar is very simple: you need to search the follow part of code in all files of the wordpress:
get_avatar($comment, 50 );
in this case the second parameter is the dimension of the avatar assuming the avatar has the width euqals to the height.
You can add a third parameter that rapresents your own images like
get_avatar($comment, 50, 'http://www.wphacks.com/wp-content/themes/HackWordPressPro/images/nophoto.gif');
Enjoy with wordpress!