How to have a specific image show up for everyone who comments?

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="https://wordpress.stackexchange.com/questions/180631/{$avatar}" class="avatar avatar-{$size} photo" height="{$size}" width="{$size}" />";
        }

    }

    return $avatar;
}

enter the new image url and paste this code in functions.php
Explaination can be found here.
how to modify avatar