This plugin creates images with scrambled letters that your visitors have to solve before they can leave a comment, or optionally get the trackback URL. It will randomly pick a random number of random letters or numbers or characters from randomly selected fonts, then make them appear randomly different, then put them on a randomly generated background.
It ships with some fonts, and you can add your own (if you want) to the qp_plugins/captcha_img_plugin/fonts/ folder.
/usr/bin/convert -swirl %rand(10,30)% -blur %rand(0,50)% -rotate %rand(0,10)% -wave %rand(1,5)%x%rand(40,120)% -background rgb(%rgb(bg)%) - -