most captchas out there can be cracked by software nowdays with enough effort.
google had a program the other year where they would show any random two users who are participating a given image, and for x amount of time they'd both type in as many possible tags for that image as they could, and the tags that they both happened to type in are associated with that image.
my idea is to use that database to show those images - as thumbnails - as captchas, and let the user type in an image tag. if it matches any tag in the database for that picture, they pass. this would have to be a captcha service provided by google.
so as not to frustrate users, we should probably include all tags typed in by both parties - not only teh ones agreed upon - presuming that information is included in the database.
in case thumbs are too small in too many cases, there could be a button to show the image full-size , perhaps in a new window, although it would be just as easy to recycle the image. or images could be shown full-size by default--perhaps in a new window, perhaps on the same page, or perhaps it could pop up just when you mouse over the thumb, like profile pictures in apps.facebook.com/yesnomaybe.