Primary options:

* We host it in the static files
 * Note that image:src still has to be a full path (so it would have to point to http://nbviewer.ipython.org)
* Link to the [smaller version](http://ipython.org/ipython-doc/dev/_images/ipynb_icon_128x128.png) in the hosted IPython docs
* Post the version we want to the website and feed from it there