gistfile1.twig

<img src="{{ post.catchimage | get_remote_image | imagine_filter('large')}}" class="img-polaroid" style="width:260px;height:260px;" alt="{{ post.catchcaption }}" />

gistfile2.php

// ...
    public function get_remote_image($url)
    {
        $tokens = explode(".", $url);
        $extension = $tokens[sizeof($tokens) - 1];
        $webdir = __DIR__ .'/../../../../../web';
        if (!is_dir($webdir .'/media/proxy')) {
            mkdir($webdir .'/media/proxy', 0777, true);
        }

        $name = md5($url);
        $relativepath = '/media/proxy/' . $name . '.' . $extension;
        $fullpath = $webdir . $relativepath;
        $result = true;
        if (!is_file($fullpath)) {
            $result = file_put_contents($fullpath, file_get_contents($url));
        }
        if ($result) {
            return $relativepath;
        } else {
            return '/images/noimage.jpg';
        }
    }
// ...