Human factors:

Google for "GD-image human" or "Keeping Robots From Stuffing Your Form"
by Randall Schwarz.  But note about blindness.

G{"robots humans images forms cgi"}

http://dbforums.com/arch/95/2003/11/963190
http://www.boutell.com/
http://www.firstproductions.com/cgi/human/
http://www.captcha.net/