The comment.tpl.php is located @<drupal-root>/modules/comment/comment.tpl.php. If you want to customize it for different content type. Copy it to your theme folder and rename it to
- comment–node-<content-type>.tpl.php
For example, the machine name of Article is article. So the file name should be comment–node-article.tpl.php.
Done =)
Reference: