It's nice idea. But it's not too hard to do it. Just replace text with images. Ez.
That's not easy bot can copy image URLs we have to encrypt images .
You also have to have a very large database of images to use otherwise a bot can easily assign a hash to each image to know what is a dog or cat or whatever. You basically have to make it difficult enough for the bot creator to not want to bother making it.
plus, you'd better add mad amount of filters to your images, swap colors, distort them, rotate by varying degrees, etc, to make hashing even harder.
keep in mind that people who know what they're doing would still be able to bypass all that, but as SparkyU said, if your system is unique AND your rewards are not worth it, then bot authors will just move along to easier targets.
btw, I can write something like that for you, but you'll have to provide your own images. and keep adding new images every day.
(see: animals on FunCaptcha to rotate, they add new animals and remove old ones all the time).