в ipb_styles.css замени на то что внутри тега body
body {
background: #D2D2D2 url('путь до фона') no-repeat;
background-attachment: fixed;
color: #5A5A5A;
background-position: center top;
font: normal 11px Segoe UI, Segoe, sans-serif;
position: relative;
-moz-transition: background .2s ease-in-out;
-webkit-transition: background .2s ease-in-out;
-o-transition: background .2s ease-in-out;
transition: background .2s ease-in-out;
}