template-extend.php

<?php

define('PLUGIN_PATH', plugin_dir_path(__FILE__));

function my_template_function($id, $size = "", $title = "", $caption = "")
{
    $template = locate_template(array('my-theme-template.php'));
    
    $found_template = ($template == '') 
                        ? $template
                        : PLUGIN_PATH . 'templates/content/latest-comic.php';
    
    // do your Flickr goodness here
    // $image = $this->api->getImage($id);
    
    require $found_template;
}

// OR:

function my_template_return_content($id, $size = "", $title = "", $caption = "") 
{
    $template = locate_template(array('my-theme-template.php'));
    
    $found_template = ($template == '') ? $template : PLUGIN_PATH . 'templates/content/latest-comic.php';
    
    // do your Flickr goodness here
    // $image = $this->api->getImage($id);
    
    ob_start();
    require $found_template;
    $found_template_content = ob_get_contents();
    ob_end_clean();
    
    // let plugins override the markup, and pass in the $image object
    return apply_filters('my_return_filter', $found_template_content, $image);
}