My bulletin board/forum is spam-bot secure. Why? Video captcha of animated
Have fun making a bot with knowledge of every manga/anime ever made with enough horsepower to OCR everything.
Every gif file has a name, filesize, hash, etc... and with a few bucks and the Mechanical Turk, I bet they can map one of these unique identifiers to the answers to your captcha, and hello spam city! I'm guessing you're really experiencing security through obscurity, which isn't real security.
1 Mole = 007 Secret Agents