django sorl.thumbnail, no module named thumbnail.models

Question

After activating the debug on sorl.thumbnail I got this error

Error importing module thumbnails.models: "No module named thumbnails.models"

and looking down on line 38 for the error callback I found this:

36  {% for article in object_list %}
37  <img src="{{article.photo.url}}" width="100px" >
38  {% thumbnail article.photo "100x100" as im %}**
39  <img src="{{ im.url }}" >
40  {% empty %}
41  <p>Thumbnail : No image</p>

I'm using the main database as cache with the following settings:

settings.py

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql', 
        'NAME': 'app',                      
        'USER': 'root',
        'PASSWORD': 'root',
        'PORT': '', 
    }
    }
.
.
.
.
CACHE_BACKEND = 'db://thumbnail_cache'
THUMBNAIL_QUALITY = 90
THUMBNAIL_PREFIX = 'images/'
THUMBNAIL_BACKEND = 'thumbnails.models.SEOThumbnailBackend'
THUMBNAIL_DEBUG = True
THUMBNAIL_KVSTORE = 'sorl.thumbnail.kvstores.cached_db_kvstore.KVStore'
THUMBNAIL_CACHE_TIMEOUT = 3600 * 24 * 365
THUMBNAIL_ENGINE = 'sorl.thumbnail.engines.pil_engine.Engine'

models.py

from sorl.thumbnail import ImageField

class Foto(models.Model):
    user = models.ForeignKey('app.User', blank=True, null=False)
    date = models.DateTimeField(auto_now_add=True)
    photo = ImageField(upload_to='%Y/%m/%d', null=True, blank=True)