If you are not using a custom template, this is what you will see
body {
font-family:Verdana,Arial;
font-size:12px;
color:#333333;
background-color:#000000;
margin:0px;
padding:0 0 50px 0;
just replace background with the code I have
background-color:transparent;
background-image:url("../images/yourimage.gif");
And it should work fine.
Always make a backup of your css file before setting changes in case something goes wrong, you can replace it.
Place your background pic in you /templates/images/ folder and change image name to your image.
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