Image based verification

[Jake]

I've been noticing that more and more sites are using the random 
alphanumeric images to verify that a real human is posting to a form 
(like NetSol.com's Whois searches, for instance). Or like this blog example:

http://blogs.officezealot.com/chris/archive/2004/08/24/2328.aspx
Does anyone have any idea how this is working? Or how the images are 
generated?

Thanks!
Jake
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