These are generally controlled in the stylesheets (stylesheet.css , for example.)
There are lots of sections where you can invoke background images, and I am assuming you want a main website background, behind the main wrappers... ?
I have one on my website, and the relevant declaration (amongst others) is in stylesheet.css
Mine says:
Code:
body {
background-image: url(../images/shadowback.jpg);
background-repeat: repeat-y;
background-position:center;
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 70%;
color: #383838;
}
The above presumes you have saved your background image in includes/templates/your-template/images
If you are unsure of my other declarations for background, read up on them at www.w3schools.com