Unfortunately, if this hack comes into common use then spambots will just defeat it by repeatedly trying to register and picking random images. Use 8 images and on average they'll succeed in about 4 tries. Captcha is immune to a simple "brute force" attack because there are too many combinations of words and letters that it can generate.
Right now this hack works because of security through obscurity - no spambots have been coded to deal with it.
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