it ( visual verification) is already worthless.

I saw a script where an operator sat in front of a screen bearing about a hundred of those images with a text box to fill them in ( the captcha text)
the submit button sent the answers back to the automated script that requested them


the only answer to that would be to have a very short timeout on the validity of the image (so when the operator keys in the answer for the robot it is already invalid) or to have some kind of 'animated game' captcha that cannot be economically automated.