[eluser]Vinzent Zeppelin[/eluser]
If the CAPTCHA image isn't showing up at all, make sure the GD extension is enabled. This happened to me.
One thing: I do need to extend the user class somewhat, in order to add a couple of fields specific to my application. I've added the appropriate columns in the user table; where are the columns handled (i.e. what do I need to patch)?