Import a class via template.php or other ways
The code needs to go in node.tpl.php, not page.tpl.php or template.php. Just follow these steps:


If you haven't already, copy the code from node.tpl.php into a file with the same name inside your theme's folder.
Edit the node.tpl.php file and add your markup around the main node div
Flush Drupal's caches (or at least the theme reigstry)


Then your nodes will all be wrapped in the markup you need.

If you just need to do this for a specific content type (I'm going to guess at one called image here), your template file will need to be called node--image.tpl.php. You'll still need the original node.tpl.php file in your theme though, as node template overrides won't work without it.