I don't think it is simple to teach a computer to recognise a little kitten on an image.
I do think it is quite simple for a computer to recognise an image that it has allready seen before. When a human has told the computer that THAT image is has a kitten on it, it is quite simple to let a computer succesfully pass this test.
Current captcha's are mostly generated images with some string on it. They will allways be different, so you can't let the computer recognise the file, it will have to interpret the contents of the image.
I don't think you are planning on generating images of kittens? or storing loads of images?
I still like the idea though, alltough I think the current generated captcha's are more secure and better capable of keeping bots out.