I don't think it is particularly standardized, but (a) sounds more natural. I would take (a) to mean you use the images themselves as a security measure to register, while I would take (b) to mean that you use the software that generates the images.
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bwI would take (a) to mean you use the images themselves as a security measure to register, while I would take (b) to mean that you use the software that generates the imagesThis 'image vs sw' usage nuance is very important to know! I would never guess it myself ....Appreciate your expert help
MUSCOVITEWhich version looks more natural to you?(a)
CalifJimDid you also notice that it's an acronym with all capitals? CAPTCHA, not captcha.OK. I will keep it in mind. Thanks!