I like the style I see here

[PinkMilk]

Admincp > Styles & Templates > Style Manager

Change Main Table Width under Sizes and Dimensions to 950

Then in Additional CSS Definitions lower down you need to add you background image css something like:

html{
background:#ff0055 url(path/to/background/image.jpg) repeat-y top center;
margin:0;
padding:0
}

This however will make you design a "fixed" design rather than what you currently have which is "fluid" which is better for todays needs with all the different devices your website maybe viewed on.