Can't save avatar field from Model method in Django

Question

I'm following this tutorial for uploading (to S3 Amazon) and manipulating an image with Django. I'm stuck somewhere, because I'm able to upload the file, but whenever the method that should create the thumb is called, I think something's missed.

This is the code:

def upload_avatar_to(instance, filename):
        import os
        from django.utils.timezone import now
        filename_base, filename_ext = os.path.splitext(filename)
        return 'media/images/avatars/%s%s' % (
            now().strftime("%Y%m%d%H%M%S"),
            filename_ext.lower(),
        )

class CustomUser(AbstractBaseUser, PermissionsMixin):
    avatar = models.ImageField('profile picture', upload_to=upload_avatar_to, null=True, blank=True)

    def save(self, *args, **kwargs):
        super(CustomUser, self).save(*args, **kwargs)
        self.create_avatar_thumb()

    def create_avatar_thumb(self):
        import os
        from django.core.files.storage import default_storage as storage
        if not self.avatar:
            return ""
        file_path = self.avatar.name
        filename_base, filename_ext = os.path.splitext(file_path)
        thumb_file_path = "%s_thumb.jpg" % filename_base
        if storage.exists(thumb_file_path):
            return "exists"
        try:
            f = storage.open(file_path, 'r')
            image = Image.open(f)
            width, height = image.size

            if width > height:
                delta = width - height
                left = int(delta/2)
                upper = 0
                right = height + left
                lower = height
            else:
                delta = height - width
                left = 0
                upper = int(delta/2)
                right = width
                lower = width + upper

            image = image.crop((left, upper, right, lower))
            image = image.resize((250, 250), Image.ANTIALIAS)

            f_thumb = storage.open(thumb_file_path, "w")
            image.save(f_thumb, "JPEG")
            f_thumb.close()
            return "success"
        except:
            return "error"

    def get_avatar_thumb_url(self):
        import os
        from django.core.files.storage import default_storage as storage
        if not self.avatar:
            return ""
        file_path = self.avatar.name
        filename_base, filename_ext = os.path.splitext(file_path)
        thumb_file_path = "%s_thumb.jpg" % filename_base
        if storage.exists(thumb_file_path):
            return storage.url(thumb_file_path)
        return ""

It seems that everything's ok, but there should be something wrong with the code.
I upload the image, no errors are caught, but:

1. The image doesn't get resized
2. No "avatar-name_thumb.jpg" is created and uploaded on the bucket

What could I do?