To get your background to repeat and be the only background go to:

/templates/crystal/css_html

Find this section:

html, body {
    height: 99.9%;
}

body {
    padding: 0px;
    margin: 0px;
    padding-top: 0px;
    background-color: #eee;
    background-position: top center;
    background-repeat: ; repeat-x (remove)
    font-family: "Trebuchet MS", Tahoma, Arial, sans-serif;
    font-size: 13px;
    line-height: 22px;
    color: #333333;
    position: relative;
    z-index: 0;
    background-image: url(../images/SpiderWebRed1.gif); (Your Image's Name)
    background-repeat: ; repeat-x (remove)


Now all I did to make my image my background for whole page is remove the repeat-x; on both background-repeats.