It's not mainly which images, it's how you select them.
CAPTCHA silently tracks your behavioral fingerprint the whole time even before taking the test (e.g. moving the mouse around the screen).
Humans generally have messy, irregular mouse paths, while a bot will most likely move in straight or highly predictable lines.
Final round of interview*
Interviewer:
Explain how CAPTCHAs can tell humans apart from bots just through image selection