title: Resizing an image using PIL in pre_save signal
body: <p>Here's my code:</p>
  
  <pre><code>from PIL import Image
  from pilkit.processors import ResizeToFit
  
  def before_saving_shop(sender, instance, **kwargs):
      img = Image.open(instance.logo)
      processor = ResizeToFit(100, 100)
      instance.logo = processor.process(img)
  
  pre_save.connect(before_saving_shop, sender=Shop)
  </code></pre>
  
  <p>I am getting this exception:</p>
  
  <pre><code>Exception Value: _committed
  </code></pre>
  
  <p>Please help.</p>