Serving dynamically generated images using Nginx and FastCGI

image nginx fastcgi

X-Accel-Redirect - exactly what you are looking for.
Usage example can be found here: http://kovyrin.net/2006/11/01/nginx-x-accel-redirect-php-rails/

Nginx is asynchronous, so it would serve all other connection without waiting data from you FastCGI script.