Sample plugin to resize any image uploaded in Joomla / JCE

uploadresize.php

<?php

defined('JPATH_BASE') or die;

class PlgSystemUploadResize extends JPlugin
{

    public function onContentAfterSave($context, &$object_file)
    {
        if (!preg_match('#^com_(media|jce)\.file$#', $context)) {
            return;
        }

        if (!isset($object_file->filepath)) {
            return;
        }

        $file = $object_file->filepath;

        if (!is_file($file)) {
            return;
        }

        // must be an image
        if (!preg_match('#\.(jpg|jpeg|png)$#', $file)) {
            return;
        }

        jimport('joomla.image.image');

        $image = new JImage($file);
        // resize image to fit 800 x 600
        $image->resize(800, 600, false)->toFile($file);
        // destroy handle
        $image->destroy();
    }
}