Вы должны определить метод save () в вашей модели:
class MyImage(models.Model):
image = models.ImageField(...)
image_crop = models.ImageField(blank=True)
def save():
super(MyImage, self).save() #will save only image, image_corp will be blank.
image_path = self.image.path #path to your non croped image
#now you can load image file and crop it usung PIL and save.
self.image_crop = 'path/to/cropped/image' #add path to cropped image.
super(MyImage, self).save() #save data.