This really only works if you have a very small site. No offense, but Alexa's rank for fofou.org is over a million. No spammer is going to spend time writing custom logic to break your captcha. I bet you don't even need a captcha -- just having custom forum software is enough to stop all automated spam.
If your site was in the top 1,000, you'd have a different experience since the spammers would be adapting to every one of your exotic captchas no matter how many times you change it.
1. fofou.org hosts the forum for my SumatraPDF site, which is 33,279 in Alexa. I think that's enough of a target, more than most forums out there.
2. I don't think it's productive to bring top 1000 websites into this particular discussion. OP's forum wasn't that popular and most forums, by definition, won't be either.
Once you're there you're playing a completely different game and at that level you should have resources to win it. I don't see spam on Wikipedia, StackExchange, Quora, HN etc. When there's will, there's a way to fight spam.
There are Human captcha breakers that you can buy for less than $10 for 1000+ captcha breaking.
There are software / plugins that provide you direct access to these people. Once the plugin detects a captcha in a form, it just passes it on to these people.
Your software solutions can do only so much. Wikipedia, StackExchange etc has Human monitoring (volunteers who report spam) in addition to automated stuff.
I think you have the advantage of hosting the forum on the low ranking site. The forum would be more tasty target if it was on (higher ranking) SumatraPDF site.
I'm responsible for helping to kill spam on Quora, so I'm happy to hear that you've had a good experience. In reality, we get a lot of spam, but we also allow anyone to edit most content so this crowdsourcing approach helps a lot. I'm sure the same is true for Wikipedia/StackExchange.
No matter how large your site, if you give people you trust the tools to stomp out spam/bad content, they will use it and you eventually "win."
If your site was in the top 1,000, you'd have a different experience since the spammers would be adapting to every one of your exotic captchas no matter how many times you change it.