Useful little piece of WordPress code, plays a particularly useful part of a dynamic featured post slideshow that I was going to detail at some point. To reference the  set upload directory url in your WordPress theme:

<?php $upload_dir = wp_upload_dir(); ?>

<?php echo $upload_dir['baseurl']; ?>

more info here http://codex.wordpress.org/Function_Reference/wp_upload_dir

Ash Mann big things and little things web design wordpress wp_upload_dir