One of the popular things today is to create a background that fills the entire browser window. To do this add the following code to your css_additional.css:
Code:
	body { 


	  background: url(images/misc/background-image.jpg) no-repeat center center fixed !important; 


	  -webkit-background-size: cover !important;


	  -moz-background-size: cover !important;


	  -o-background-size: cover !important;


	  background-size: cov
...