Some users, like me, are hosted on IIS servers and our hosters can be very strict about using a htaccess file. Contact Form 7 in combination with really simple captcha won't work for (some of) us because it'll generate a htaccess file in /wp-content/uploads/wpcf7_captcha/ which will be detected and blocks the use of that folder.
So, if anyone runs in to the problem of captcha images not showing while on IIS servers, the generated htaccess file could be the problem.
Thanks for the great plugin. I know generating the htaccess files is done to guarantee some safety but for some users on IIS it just won't work. Could you include an option in wpcf7 or maybe a warning for users on IIS? (If you want to do some testing, I'll set up a clean WordPress install and give you access to it)