Multiple images
I recently saw a system that used six images in a 2x3 array to form the characters displayed. It only worked if all 6 images were up there. Yes, you can still use some 12 year old making 10 cents an hour in some fourth world country to solve it, but it makes the computed solution very difficult.
Using the pr0n suffer solution depends on having someone handy (pun not intended) at the time.
But ...
Let's do the math. You pay a kid 10 cents an hour. He/She solves 60 Captchas an hour. That's 60 drones who each send 5000 emails before getting shutdown (I'm being generous to Gmail). If they get a .1% hit rate at $25 profit, that's $7499.90 after operating expenses. But let's call it .01%. That brings it down to $749.90. And since you're "working" 24x7, that turns into a boat load of money. I'm in the wrong business. And that's from one kid (probably 2, two tweleve hour shifts, I'm a benevolent employer).