While probably not 99% reliability, I think google software could often solve this one too: Reverse image search could provide an image description, which combined with number of search queries for eg 'unsafe $image_description' to give a ranking. Google can already mostly handle synonyms; it does not need a mastery of the area as it can just use correlations and usage patterns.
Also, being solvable for humans and not computers is not the only criteria for a captcha. For example it must be quick and relatively easy for humans, and provide a definitive consistent answer. The brilliance of recaptcha is that it was making the work that humans excelled at actually useful beyond just 'proof of humanity'.
Also, being solvable for humans and not computers is not the only criteria for a captcha. For example it must be quick and relatively easy for humans, and provide a definitive consistent answer. The brilliance of recaptcha is that it was making the work that humans excelled at actually useful beyond just 'proof of humanity'.