JPEG quality/compression on thumbnail and resized images

I'm probably missing something obvious, but I don't see how one adjusts the amount of JPEG compression used on thumbnails and resized images. Based on the file sizes I'm seeing, I'd guess that the graphics engines are using the highest setting. Further, I'm guessing the graphics engine is choosing this option since my source JPEG files are set to the highest quality in my camera. Is there a way to force a particular value?

What about making the compression step into a graphic rule? As it is now, I would think multiple JPEG compressions would have a fairly negative effect on the image quality since each iteration is lossy (I think, but I'm definitely not a JPEG expert).

/**
* Compress an image based on the image_quality setting. The options parameter is ignored.
*
* @param string $input_file
* @param string $output_file
* @param array $options
*/
static function compress($input_file, $output_file, $options) {
if (!self::$init) {
self::init_toolkit();
}

Image::factory($input_file)
->quality(module::get_var("core", "image_quality"))
->save($output_file);
}