I'm afraid there is no easy way to use another Captcha implementation than the current one.
We decided to go with reCAPTCHA since it is run by the original creators of CAPTCHA and has the highest security standards. Many other implementations of CAPTCHAs can be easily broken.
Other implementation also requires GD image library, which not all hosting support it.
However, I do realized that reCaptcha won't be perfect for everybody.
Regarding the looks and feel of the CAPTCHA, actually you could customize it to suit your site.
The default color is red, but you could change it to 'white','blackglass','clean' and 'custom'.
I haven't put any documentation regarding this yet, but here's how:
Open your hooks/custom_hooks.php file, you'll see sample hooks code. Particularly this one:
function form2_hook_pre_captcha(){
$capthca_theme =<<<EOT
<script>
var RecaptchaOptions = {
theme : 'blackglass'
};
</script>
EOT;
return $capthca_theme;
}
This function is being used to customize your captcha color.
Uncomment that function and change the themes with the value I mentioned above.
Also, change the fuction name to use your form id.
So for example this form http://www.appnitro.com/demo/view.php?id=7 has form id 7. The code would be:
function form7_hook_pre_captcha(){
$capthca_theme =<<<EOT
<script>
var RecaptchaOptions = {
theme : 'blackglass'
};
</script>
EOT;
return $capthca_theme;
}
I hope I'm being clear for you. Otherwise feel free to ask.