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This is something I might be willing to work on, if someone could locate some open-source or public-domain code for making the image.
I know how to use the GD library to make an image that contains a string of text, but for maximum effectiveness, the text should be "distorted" so that it can be read by a human, but not by an image-processing script. I've looked around, but haven't found anything.
So that is your mission. Find me some code.
P.S. PHP code is preferred, but something that could be translated into PHP might suffice, for example Perl, C, or just an algorithm.