[blechschmidt]

A possibility to break these might consist in using Google's image reverse search and check whether the word which you are supposed to identify images for appears on the result pages. However, the Google search is protected by reCaptcha - so you could build your own image reverse search or you use one implementing captchas which are easier to break.

Otherwise, if you own a highly frequented website, you could ask your users to solve the captchas for you by proxying the challenges to them.