docs: def clean_picture(self):
          """
          Validates format and file size of uploaded profile picture.
          
          """
          if self.cleaned_data.get('picture'):
              picture_data = self.cleaned_data['picture']
              if 'error' in picture_data:
                  raise forms.ValidationError(_(u'Upload a valid image. '
                              'The file you uploaded was either not an image '
                              'or a corrupted image.'))
                  
              content_type = picture_data.content_type
              if content_type:
                  main, sub = content_type.split('/')
                  if not (main == 'image' 
                              and sub in defaults.ACCOUNTS_PICTURE_FORMATS):
                      raise forms.ValidationError(_(u'%s only.' % 
                                      defaults.ACCOUNTS_PICTURE_FORMATS))
          
              if picture_data.size > int(defaults.ACCOUNTS_PICTURE_MAX_FILE):
                  raise forms.ValidationError(_(u'Image size is too big.'))
              return self.cleaned_data['picture']