CustomPress - how to add content as background image

So obviously I don't understand how the shortcodes work with this great plugin, but am hoping a guru has a simple answer to this one.

I have created a CustomPress Customfield which allows me to upload an image. I want to use this image as a background element within the Theme I am hacking. How/where is the best place to pull that image into the <div> within the loop?

Thus far I can see the image in the Media library and can link to the image, so I know CustomPress is doing it's thing. I can use the usual Shortcode information to see the content within the post, but I cannot figure out what I need to do include it into something like this:

background-image:url( <?php echo do_shortcode('[custom_fields_block] xxxxxx [/custom_fields_block]'); ?> );