Having different image behind page content on each page

Hi all. I’m pretty new to wp and am building a child site using the twenty twelve theme. i have figured out how to do most of the changes that i want but i can’t figure out how to have a different image behind the content on different pages. not the background image, i know how to change that, but the body background.

i have this code in my css file

body .site {

background-image:url(‘main_background.jpg’);
padding: 0px;
margin-top: 0px;
margin-bottom: 0px;
box-shadow: 0 0px 0px rgba(0, 0, 0, 0);
}
which shows that image on all pages but i only want that on the home page and different image on all the rest. All of my pages are static info.

i know there are probably 3 different ways to do it but i keep getting stuck!

Any help would be much appreciated,
Josh