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// add a new default avatar to the list in WordPress admin
// change path to reflect where your default image lives
add_filter( 'avatar_defaults', 'mytheme_addgravatar' );
function mytheme_addgravatar( $avatar_defaults ) {
    $myavatar = get_bloginfo('stylesheet_directory') . '/_/images/avatar.gif';
    $avatar_defaults[$myavatar] = 'New Default Gravatar';
    return $avatar_defaults;
}