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Here's how one of them works:
Original form is requested. A unique key is created and stored against the user (cookie, session, etc). That key contains the text written to the image. When the form is submitted, the text the user entered from the image is encrypted the same way the original key was encrypted, and then compared against what was originally stored against the user.
Not saying that's how they all work, but at least one of them I've seen.
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