The site's logic is nearly identical to this gist: use ImageMagick to do the rotate, noise, etc.

How is a web page app better than a shell script you can run yourself? Of course, any old port in a storm – if you can’t run a shell script, this is better. But if you can run things yourself, you should. A shell script will stay on your file system and not disappear in a couple of years when the original author either gets bored with it or decides to make it proprietary.