Dynamically create image thumbnails (using django)

Question

I would like to dynamically create thumbnails based on parameters in the URL. For example, http://mysite.com/images/1234/120x45.jpg would create a 120x45 thumbnail for image id 1234.

The obvious solution to this is to have a django view which does the following:

1. Look for a previously cached version of the image at this size.
2. Create a thumbnail if it's not cached (some logic for locking so that only 1 process creates the thumbnail and other processes wait).
3. Pipe the results through django.

That should "work", but I'm concerned about performance. I don't like the idea of using django to serve static content. What are some other ways to accomplish this problem?