Help with my image plugin

Hi, I’m trying to write a plugin and I’m a bit stuck. The plugin get’s images from posts with a certain class.
Here is my code:

<?php
/*
Plugin Name: Get The Image
*/

add_filter('the_content', 'ed_get_image');

function ed_get_image($content)
{
       if(preg_match('!<img(.*?)class="imagetest"(.*?)/>! iU', $content, $matches))
       {
               return $matches[0];
       } else {
               return false;
       }
}

?>

The problem I have though is that my plugin is always on. How do I make it so that people have to insert the function into the template first before it is used?
E.g. They’d have to put something like: ‘<?php ed_get_image(); ?>’ in their template before the plugin is used.