This is not simply a checkbox more or less... it's hardcore theming ;-)
You should create a template.php-file (inside your theme-directory) in which you can override functions. These functions determine what will be put in the submitted by-line. The one you would like to change. In your template.php-file you put the following:
<?php
function [theme-name]_node_submitted($node) {
return t('Posted by !username',
array(
'!username' => theme('username', $node),
));
}
Comments
A bit technical
This is not simply a checkbox more or less... it's hardcore theming ;-)
You should create a template.php-file (inside your theme-directory) in which you can override functions. These functions determine what will be put in the submitted by-line. The one you would like to change. In your template.php-file you put the following:
Replace [theme-name] with the name of your theme.
But first: you have to create your own theme: http://drupal.org/theme-guide/6