You have no idea how spam works. A bot isn't just one user trying to enter "cat" repeatedly. Botnets send requests from thousands of different IPs. You wouldn't know which ones are real users, and which ones are bots.
My spam script would always answer "cat", so among 8 options (cat, dog, house, drum, road, tree, book, horse), I'd get a 12.5% success rate.
Plus you constantly have to update your image library, which a huge pain.
Also, recognizing 10,000 images will take me around one day and less than $1000 with Amazon turk, thus giving me a perfect 100% success rate. After that you would have to completely renew your image database.
That's horrible. Instead of drawing graphs on the client side, you're pushing insanely large number of images to the browser. It makes sense for google maps, not for graphs.
Plus you can't do any animation, changes to the graphs on the fly.
Notice how that the chart scale doesn't start with zero. In reality they win maybe 10-15% in speed, but they have all the crap overhead to support and manage.
Guys, cmon, this is obvious book spam. There's even an affiliate link at the bottom