Great :), first impressions:

I've seen a few of these before and the things I remember I liked about them was the ability to provide a default image incase you cannot load the image.

Major kudos if you could do a g_face so you could center / crop around faces!

You say you provide a CDN and yet the images are loading slow under HackerNews weight. I guess the type of people who are going to use these aren't super concerned about being hammered!

Lastly the Heroku add-on, does that mean I can run your code on my Heroku instance instead of rely on your site? That would be fantastic!