In this theme, the “skin” option from Appearance > Theme Options controls the background of your website. However, you can go to Appearance > Background and setup your own custom background. When you do this, the theme will use those background options to override the the skin option from your theme options page.
So, if you’ve got a modified version of the theme’s slate grey background image, you’d just upload it and apply it through Appearance > Background.
Here is a PSD source file of all the background skin colors/textures:
And you can find all of the current background images here in the theme:
All of the images are named as