title: django-stdimage not saving the image
body: <p>I'm using django-stdimage and using a ModelForm. When submitting the form it saves without a problem. The image urls are stored but no image files are stored in the uploaded folder or any other folder. Why could this be?</p>
  
  <pre><code>import datetime
  from django.db import models
  from django.forms import ModelForm
  from ext.stdimage.fields import StdImageField
  
  from south.modelsinspector import add_introspection_rules
  add_introspection_rules([], ["^ext\.stdimage\.fields\.StdImageField"])
  
  class Post(models.Model):
      image = StdImageField(upload_to='uploaded', thumbnail_size=(250, 250))
      date = models.DateTimeField(auto_now=True)
      description = models.TextField(max_length=20000, blank=True)
  
  class PostForm(ModelForm):
      class Meta:
          model = Post
          exclude = ('date')
  </code></pre>