Good point nina, my solution perhaps approached the issue from too much of a beginner standpoint, like someone who is not familiar with style sheets, as many of my customers are.

As far as editing the stylesheet of the default skins, nina is absolutely correct, the pagemaster and skinmaster classes are what current sets the background-color.  You can change these settings by creating a custom skin or override them by going into Admin - Site Settings and scrolling down and expanding the Stylesheet Editor section, adding the following lines and clicking 'Save Style Sheet'

.pagemaster{background-color:transparent;}

.skinmaster{background-color:transparent;}

This will allow you to set the Background Image in Admin-Site Settings.  If you wanted to set the background via the style sheet itself, you could add the following lines instead:

.pagemaster{background-color:transparent;background-image:url(bgimage1.jpg);}

.skinmaster{background-color:transparent;}

Where bgimage1.jpg is the url or file and path of your background image relative to your corresponding skin html file.  I recommend using relative paths instead of fully qualified urls (which hardcode http) to prevent issues with your skin causing mixed content errors if it is used on an SSL secured (https) page.