This is purely guessing, but checking regular images (right size, format, ...) will be the "day-to-day" business of looking after a website. And more often than

This is purely guessing, but checking regular images (right size, format, ...) will be the "day-to-day" business of looking after a website. And more often than not the administrator will therefore "Open image in new tab..." and become aware of an incorrect image path. The Captchas, on the other hand, might have been using an existing software. Remember: These captcha images will have to be autogenerated by a script which, as a convenience to the user, might have used some kind of mechanism to determine "fully qualified" URLs. And this had slipped below the radar, as it's a feature used much less often, and hence likely to receive much less scrutiny. I think it's pretty likely that these kinds of information leaks can happen when you deal with a larger codebase or system. Hence following the advice of some other HN users, who recommend a strictly firewalled system for this kind of use-case, looks like a prudent thing to do.