Use default Captcha
If you would like to use JCaptcha in your application quickly, you should use the DefaultManageableImageCaptchaService, as described in the 5 minutes application integration tutorial.
Use an other CaptchaEngine
If the default generated captcha doesnt answers your needs, you could try the out-of-the-box engines provided by the JCaptcha distribution.
The list of engine is available in the Javadoc.
You could take a look at generated captchas in the Samples tests page.
In order to use a specific Engine, you have to provide it in argument of the CaptchaService constructor.
Customize a CaptchaEngine
The third way to change the JCaptcha rendering configuration is to implements a custom CaptchaEngine.