So here's a way to create thumbnails at upload with LiipImagineBundle. The trick is to use some of their other services:
/**
* Write a thumbnail image using the LiipImagineBundle
*
* @param Document $document an Entity that represents an image in the database
* @param string $filter the Imagine filter to use
*/
private function writeThumbnail($document, $filter) {
$path = $document->getWebPath(); // domain relative path to full sized image
$tpath = $document->getRootDir().$document->getThumbPath(); // absolute path of saved thumbnail
$container = $this->container; // the DI container
$dataManager = $container->get('liip_imagine.data.manager'); // the data manager service
$filterManager = $container->get('liip_imagine.filter.manager');// the filter manager service
$image = $dataManager->find($filter, $path); // find the image and determine its type
$response = $filterManager->get($this->getRequest(), $filter, $image, $path); // run the filter
$thumb = $response->getContent(); // get the image from the response
$f = fopen($tpath, 'w'); // create thumbnail file
fwrite($f, $thumb); // write the thumbnail
fclose($f); // close the file
}
This could also be done by directly calling the Imagine library functions if you had no other reason to include the LiipImagineBundle. I'll probably look into that in the future, but this works for my case and performs very well.