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)%) - -
This plugin includes Liberation Fonts, and you can add your own if you want to. You add TrueType font file (*.TTF) to the specified fonts folder which is typically /fonts/ in this plugin. It would be best to select fonts which are not difficult for the average person to read before being "captchad" because people will not be able to post comments until they solve your captcha.
Some free font sites, and I'm sure you can find others if you search a bit: