Be lazy when clicking the images. Mailboxes are parking meters.

Look if the site has an option to use the noscript recaptcha (you might be able to force it to appear by blocking scripts). That one's much easier: as long as you get it right it lets you through, and you always have to select three squares (none of that "keep clicking until there's no more matching" garbage).

The first set is all you really need. Answering the second set (if presented) with completely wrong answers will usually cause you to pass. Wild guess -- perhaps ReCAPTCHA is checking for mouse movement and other behavioural heuristics after the initial test.

Seems like they are afraid of robots.

The traffic cone was particularly heinous as I am damn sure they mislabeled one of the images.