I already prevented bots from posting comments by using image code. However, they can still register. Is it possible to use image code (which seems to be the best protection) in the user registration form?
Yes, using the same technique (the captcha plugin) that you already appear to use. Please read the documentation that comes with the captcha plugin. If that doesn't help, do as suggested per board rules and post the needed details: posting a link to your gallery is mandatory. Saying what version of captcha (plugin vs. mod) you are using and which version of the plugin or mod might help as well.