Function to get wordpress theme URL

This function will return the theme directory URL so you can use it in other functions:

get_bloginfo('template_directory');

Alternatively, this function will echo the theme directory URL to the browser:

bloginfo('template_directory');

So an example for an image in the themes images/headers folder would be:

<img src="<?php bloginfo('template_directory'); ?>/images/headers/image.jpg" />