By the way:
add this to your functions.php:
/**
* add a default-gravatar to options
*/
if ( !function_exists('fb_addgravatar') ) {
function fb_addgravatar( $avatar_defaults ) {
$myavatar = get_bloginfo('template_directory') . '/images/gravi1.png';
$avatar_defaults[$myavatar] = 'gravi1';
$myavatar2 = get_bloginfo('template_directory') . '/images/gravi2.png';
$avatar_defaults[$myavatar2] = 'gravi2';
$myavatar3 = get_bloginfo('template_directory') . '/images/gravi3.png';
$avatar_defaults[$myavatar3] = 'gravi3';
$myavatar4 = get_bloginfo('template_directory') . '/images/gravi4.png';
$avatar_defaults[$myavatar4] = 'gravi4';
return $avatar_defaults;
}
add_filter( 'avatar_defaults', 'fb_addgravatar' );
}
This has the result, that in your Wordpress settings more own avatars will be shown for choosing them. No need to edit anything else.
You only need to add the images to the images folder of the theme.
In your Wordpress settings it will look like this:

This is a suggestion of bueltge.de and it works perfect:
http://bueltge.de/avatare-zum-standard-von-wordpress-hinzufuegen/852/