If you draw the labels onto the image (make them each one word) and make the user type them, it's still an easy captcha for a person, but much harder for a bot.

If this spreads, it will be worth making a bot for, so that you can spam any of the sites that use it -- right now that bot just needs a few hits on average, and a slightly smarter bot can space them out, so that they are conceivably like commenters.