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On a standard XOOPS 2.3.x install (i.e. you haven't installed the bundled Profile module), the captcha image appears on the second page of the registration process.
I can't answer your query if you are using the Profile module as I don't use it myself. I'm sure that someone else will be along shortly to answer this bit!
PS: I've deleted the the other thread that you started on the same subject; no need to double post