[mikejarema]

I think a good chunk of the images are captured by way of Google's Streetview vehicles [1]. I'm seeing blurry images of house and apartment numbers all the time. So I'd imagine there are always new images popping up that Google can feed into the recaptcha system that haven't been seen before.

[1] http://www.google.com/recaptcha/intro/#creation-of-value

[egsec]

Correct, I am referencing the new nocaptcha system. Those images would get stale as opposed to those in the traditional scanned book, street signs, house numbers in the recaptcha.