I was just about to post that, damnit. :-(
http://www.zend.com/zend/tut/dynamic.php
I didn't know mod_rewrite what it is called though. I just realized that's what's happening.
Here's the mod_rewrite page:
http://httpd.apache.org/docs/mod/mod_rewrite.html
[also]
If you'll notice, it's not a jpg, it's a PNG. You can also put any number of letters/characters/whatever after the jpg and it will work.