I think this should be a mod or even standard on SMF.
Catchpa codes have been getting more tricky and longer 7-10 characters.
Theres a way to show the image perfectly clear and the bots can read exactly what the image says but they will put in the wrong answer.
Well I'm sure this idea is not original but here it goes...
Instead of asking the user to input what the image says. Have the image show a simple math problem and ask the user to solve it.
ex.
The image showes "100+32" but the correct code is "132" easy on the eyes of the user and prevents automated registration.
also you could have the image show "what are the next 3 letters after D" and the answer would be "EFG"
To vary from numbers to letters.
It'd take just as long to "crack" as any other CAPTCHA system. Learn the questions, input the algorithm, and crack it. There are other methods, search the forums and you might find a few.