NGINX redirect attempts download instead of rendering image

According to nginx pitfalls, it's slightly better to omit the unnecessary capture, using $request_uri instead. In that case, append a question mark to prevent nginx from doubling any query args.

server {
    listen      80;
    server_name signup.mysite.com;
    rewrite     ^   https://$server_name$request_uri? permanent;
}

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.