我可能已经找到了解决办法。这似乎有用，但也许我错过了什么？

function custom_user_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 );
    } else if ( 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 = sprintf( \'%s/images/avatar/support-male.png\', get_stylesheet_directory_uri() );
            $avatar = "<img alt=\'{$alt}\' src=\'{$avatar}\' class=\'avatar avatar-{$size} photo\' height=\'{$size}\' width=\'{$size}\' />";
        }
        else if ( $user->data->ID == \'200\' ) {
            $avatar = sprintf( \'%s/images/avatar/support-female.png\', get_stylesheet_directory_uri() );
            $avatar = "<img alt=\'{$alt}\' src=\'{$avatar}\' class=\'avatar avatar-{$size} photo\' height=\'{$size}\' width=\'{$size}\' />";
        }
        else {
            $avatar = sprintf( \'%s/images/avatar/default.png\', get_stylesheet_directory_uri() );
            $avatar = "<img alt=\'{$alt}\' src=\'{$avatar}\' class=\'avatar avatar-{$size} photo\' height=\'{$size}\' width=\'{$size}\' />";
        }
    }
    return $avatar;
}
add_filter( \'get_avatar\' , \'custom_user_avatar\' , 1 , 5 );