Ok, that is weird, I hadn't considered that it might be at the client end. I have tried with a clean version of chrome and I get the same as you do, in Firefox however the picture does not show. I will check if it is maybe some add-on? Any hints what it might be or where errors are maybe logged in FF? Javascript or something? I am hoping that this is then just due to my atypical setup and that most other people do not get this problem. I will also check on this site if I get it on all CBcaptcha or just on mine.
edit: so I logged out of joomlapolis.org and clicked register, and here is the captcha! So it is something with FF and my site.
Thanks for the help,
Dennis