Com security image with be shipped soon with a patch for Joomla 1.03 which also modify the login and registration process, see screenshots below....
in module login:
in component registration
in component registration, lost password
The more background, the more fonts, the difficult for a robot to do an OCR on pictures....I was thinking of doing automating search on Google images and randomize the background but Google forbid such kind of misuse of thiers services (I understand that)