I didn't find a solution for my problem: I wanted my new blog to be protected against robots and I want it accessable. Therefore I created a small hack to have own captchas and combined it with some quiz like 'find the missplaced word' in a line like 'Istambul Paris London Coffee Rio'.
The captchas are created with imagemagick. And I think they are secure enough, even if I didn't work with backgrounds. There are at the moment 3 fonts in use. I don't use mysql for it, I just check the filemtime($filename); and delete the captcha after a successfull post.
The Quiz part has at the moment 20 lines with each 5 to 10 words, it's easy to extend it.
It works so far but it is not advanced enough to publish it as a plugin for wordpress.
My question is: Is the idea good enough to spend more time to write proper code?
I use the code in my new german blog at garps.com. A test is availible at garps.com/bam.php