If that is the best attack on my idea, then I'd say that's pretty good! A spammer would have to target the site specifically (for tailor-made script) and parse an animated gif into individual images, then send every image into Tin Eye (which is awesome but the API isn't free right?).

That said, I am a complete amateur at captcha design. Cool research though and I bet a lot of HN'ers would be interested in your PoC if you haven't already submitted it.