The way I achieved this on a site I built was as follows... (BTW the site was set to be a fixed width at 900px) but the footer extends across the entire width of the screen.
In the ATA-->Add HTML/CSS Inserts. Add the following
background-image: url('<?php bloginfo('template_directory'); ?>/images/custom/footerbg.jpg');
background-position: bottom left;
I used a .jpg that was a simple gradient. It works perfectly!