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.