The captcha images with the letters are to difficult to read and I have not found any tutorial how to for example force it to have only number or capital letters. Maybe someone knows where to find this.
You can change the character set used in the CAPTCHA in the configuration file (/components/com_jcomments/libraries/kcaptcha/kcaptcha_config.php). But it will reduce the resistance of CAPTCHA image to OCR.
Thanks for the Review. Please open a thread on our support forum where we can discuss your requested feature in more detail. There are Pros and Cons to be highlighted with regard to implementation of opacity across browsers etc ...