Theming comments in drupal 6
Adding the user´s picture in the comment post, it has to be enabled from the theme settings, admin>theme>myzen>configure. If a default user picture (like an icon in my case) is to be used for non authenticated users posting comments, a default path has to be created in admin>user settings, and then enter the path to a previously uploaded image (in my case: /Scoutsdrupal/sites/default/files/userpics/user.png.
If markup needs to be themed the comment.tpl.php file has to be duplicated into the theme folder for customization, but I found that served my purpose and there was no need for that. Only styles were added in the stylesheet.