I am trying to set a background image to be outside the actual containing div.
<div class="expandable">Show Details</div>
background: transparent url('./images/expand.gif') no-repeat -20px 0px;
You're going to have to put the background image inside a separate element. Background image positions cannot place the image outside the element they're applied to.
edit your question jogged my memory and I went and checked the CSS specs. There is in fact a "background-attachment" CSS attribute you can set, which anchors the background to the viewport instead of the element. However, it's buggy or broken in IE, which is why I've got it sitting on the "do not use" shelf in my head :-)
edit — Note that this answer is from 2010, and newer (and, more importantly, widely-supported) CSS capabilities exist in 2016.