How to serve cached images in django sorl-thumbnails ? It's raising 404s

How to serve cached images in django sorl-thumbnails ? It's raising 404s

I'm new to sorl-thumbnail and I am generating thumbnails that behave like
they should work fine but they don't. Here's an example of what I'm doing
{% thumbnail
"http://www.tlswebsolutions.com/wp-content/uploads/2012/03/django-logo-negative.png"
"200x200" as im %}
<img src="{{ im.url }}">
{% empty %}
<p>No image</p>
{% endthumbnail %}
I'm using memcached with the default configuration and the stats look like
my thumbnails are actually stored in the cache. The thumbnail appears in
the my folder /proj/myapp/cache/x/xx/xx.jpg
But the site raises a 404. Which I think is normal, as there is no URL
pointing to that /cache directory.
I checked the doc, they don't seem to say that there is anything
particular to do about this. Did I miss an obvious step somewhere ?
Thanks a bunch!