gistfile1.php

<?php

//...
if ($this->Video()->exists()) {
    $basePath = Director::baseFolder() . DIRECTORY_SEPARATOR;
    $folder = Folder::find_or_make('video'); // relative to assets

    $relativeFilePath = $folder->Filename . 'video-' . $this->Video()->ID . '-thumb.jpg';
    $fullFilePath = $basePath . $relativeFilePath;

    if (!file_exists($fullFilePath)) {
        $fp = fopen($fullFilePath, 'w');
        $ch = curl_init($this->Video()->ThumbnailURL);
        curl_setopt($ch, CURLOPT_FILE, $fp);
        curl_exec($ch);
        curl_close($ch);
        fclose($fp);
    }

    $image->setFilename($relativeFilePath);
    return $image;
}
//...