In your stylesheet.cs file the first entry is
Code:
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #664826;
background-color: #FFBBD5;
}
Create/upload your image to your template/images folder and edit the above to include:
Code:
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #664826;
background-color: #FFBBD5;
background-image: url(../images/your_new_bg.jpg);
background-repeat: repeat; add-x or -y depending on the image
}