add_filter( \'get_avatar\' , \'my_custom_avatar\' , 1 , 5 );

function my_custom_avatar( $avatar, $id_or_email, $size, $default, $alt ) {
    $user = false;

    if ( is_numeric( $id_or_email ) ) {

        $id = (int) $id_or_email;
        $user = get_user_by( \'id\' , $id );

    } elseif ( is_object( $id_or_email ) ) {

        if ( ! empty( $id_or_email->user_id ) ) {
            $id = (int) $id_or_email->user_id;
            $user = get_user_by( \'id\' , $id );
        }

    } else {
        $user = get_user_by( \'email\', $id_or_email );   
    }

    if ( $user && is_object( $user ) ) {

        if ( $user->data->ID == \'1\' ) {
            $avatar = \'YOUR_NEW_IMAGE_URL\';
            $avatar = "<img alt=\'{$alt}\' src=\'{$avatar}\' class=\'avatar avatar-{$size} photo\' height=\'{$size}\' width=\'{$size}\' />";
        }

    }

    return $avatar;
}