In an absolutely brilliant but evil move, a Trojan fools users into solving CAPTCHA images. Infected users think that they’re entering codes to see a model undress, when actually they’re helping crackers register for illegal Yahoo accounts.
A quick defence measure for this would be to incorporate the site’s brand into the CAPTCHA image itself. The branding would need to be slightly randomized and overlap the random part of the image, but at the very least this will make it more difficult for to disguise their true intent.