ja.son-williams.ca/www/plugins/jasonwilliams/imgcache/Plugin.php

<?php namespace JasonWilliams\ImgCache;

/**
 * The plugin.php file (called the plugin initialization script) defines the plugin information class.
 */

use System\Classes\PluginBase;
use JasonWilliams\ImgCache\Classes\Image;

class Plugin extends PluginBase
{

    public function pluginDetails()
    {
        return [
            'name'        => 'Image Cache',
            'description' => 'Saves external images locally and presents them via a twig filter.',
            'author'      => 'Jason Williams',
            'icon'        => 'oc-icon-cloud'
        ];
    }

    public function registerMarkupTags()
    {
        return [
            'filters' => [
                'imgcache' => function($imgurl) {
                    $image = new Image($imgurl);
                    return $image;
                }
            ]
        ];
    }
}