While interesting, this may not be useful in all cases. So I'd prefer if we could make this optional. How about a config parameter `enableCssImageFingerprinting` (or similar)?

Another thing: I'd put the logic here into a dedicated method, so it's easier to maintain. Something along:

```php
if($this->enableCssImageFingerPrinting) {
    $this->addCssImageFingerPrints($f);
}
```