Serving resized image files

Another solution could be to have nginx proxy to itself. Set up two upstream vhosts, one which serves your normal images and one which serves trollface.jpg only.

Include the normal vhost as your upstream server with a weight of 9 and the trollface vhost as the other upstream server with a weight of 1.

This will serve the alternate image once in every 10 requests but not once in every 10 requests for each individual file.

Alternatively, the HttpEchoModule might be able to do the trick for you. A combination of echo_subrequest_async and echo_random could give you a 1 in 10 chance of delivering trollface.jpg instead of the file they requested.

There is also this post on the mailing list which suggests that someone is working on per-IP-address counters in the nginx config.

Note that I am not recommending any of these solutions, just noting that they're possible.

Try enabling caching of all files served, by setting the frequentHitThreshold to 1 (instead of the default value of 2 consecutive requests within 10 seconds). This should take some load off of the disks if that's your bottleneck. For more info on setting this attribute, see: http://www.iis.net/ConfigReference/system.webServer/serverRuntime

You might also wan't to configure HTTP Compression for static content like this: http://technet.microsoft.com/en-us/library/cc754668(WS.10).aspx