I'd probably use just use Tor or similar to make all requests come from different IPs. You don't need to increase the number of pictures by a couple, you need t

I'd probably use just use Tor or similar to make all requests come from different IPs. You don't need to increase the number of pictures by a couple, you need to increase the number of possibilities by several orders of magnitude. A traditional captcha with 6 letters/numbers has 2,176,782,336 possible combinations. Not 36. The whole point of a captcha is that there is a large enough set of possibilities such that you can't just get it right by guessing randomly a few times.