I am a total noob, but I just figured it out. Here is what you do to change the background image of the page:

1) Go to the site settings page

2) Scroll down to the bottom and open up the Stylesheet editor

3) Add this line to the style sheet

.pagemaster {
background-image: url(your_image.jpg);
}

Here is an example of what it looks like after it is inserted:
-------------------------------------------------------

/* ================================
    CSS STYLES FOR DotNetNuke
   ================================
*/  
/* PAGE BACKGROUND */
/* background color for the header at the top of the page  */

.pagemaster {
background-image: url(side_repeater.jpg);
}
.HeadBg {
}
/* background color for the content part of the pages */
Body
{
}
Etc. Etc. Etc.....

4) Dont forget to save the stylesheet using the link

5) Press the Update link to update your changes on the page.