Hi Gawain,
I was installing bolt via composer so it is sits under ``/vendor`` folder.
This is my ``/web/index.php``:
```
<?php
if (preg_match ( "~^/thumbs/(.*)$~", $_SERVER ['REQUEST_URI'] )) {
	// If it's not a prebuilt file, but it is a thumb that needs processing
	// define('OPTIPNG_ENABLED', true);
	define ( 'FILE_CACHE_DIRECTORY', __DIR__ . '/../cache/thumbs/' );
	require __DIR__ . '/../vendor/bolt/bolt/app/classes/timthumb.php';
} else {
	require_once ('../src/bootstrap.php');
	// Here we go!
	$app->run ();
}
```
Thumbnails are working as expected now for me.