[mod_python] How do I serve dynamic images?

I've searched the web but have only seen code to serve dynamically 
generated images in the following way:
	buff = StringIO.StringIO()
	img.save(buff,...)
	req.write(buff.getvalue())
and setting the appropriate html headers.  This is fine if I want to 
serve the image in its own page.

But how do I serve dynamically generated images inside an <img> tag?

* note: I don't want to create the image on disk first, I wish to only 
serve it directly from python in memory.