Backround help

Here is the code I have,

    body {
       color: #000;
        font-family: 'lucida grande','Lucida Sans Unicode', tahoma, sans-serif;
        font-size: 75%;
        line-height: 1.7em;
        background-color:#FFFFFF;
       margin: 0;
       padding: 0;
       text-align: center;
       font-size: small;
    }

And I i wanted to make a image background I do this

    body {
       color: #000;
        font-family: 'lucida grande','Lucida Sans Unicode', tahoma, sans-serif;
        font-size: 75%;
        line-height: 1.7em;
        background-image:url('LINK');
       margin: 0;
       padding: 0;
       text-align: center;
       font-size: small;
    }

If I add a background image it goes behind my discussions, which I don't like, should either

Figure out the Pixels in the entire website width and remove those from background

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