I looked a bit for a tutorial on it too, but there isn't much to be found as far as people writing step by step directions on making an image verification.
one of the members here has a short tutorial on his site about images with random text at this page.
since you brought this up and i just finished my last project, tonight i started a portable class for validating images that will be very easily integratable into all forms, and i hope to add lots of options for adding colorful and skewable text. so once i get a production done, ill post back. at this point ive got it to the point of saving the data, next ill work on the interface to validate the code a person entered.