I like how you are thinking outside the box a bit. Having said that, there are a couple of issues: 1) This is really not a hard problem for a computer to solve

I like how you are thinking outside the box a bit. Having said that, there are a couple of issues: 1) This is really not a hard problem for a computer to solve. Detecting different image types is pretty trivial. You made it a lot easier by making the letters sit in the boxes in a regular manner. 2) A high percentage of captchas are now broken by humans, instead of robots. This is not a problem you will likely encounter in a small site, though. (1) Is an easier problem to solve. You could do "input the letters near the cat", or "input the letters near the map of florida". (2) Is harder, but most of the people who break captchas live in 3rd world countries, so you could possibly arbitrage cultural knowledge (see florida example above). They will catch on pretty damn quick, though.