function fillBackground() {
    var de = document.documentElement;
    var w = window.innerWidth || self.innerWidth || (de && de.clientWidth) || document.body.clientWidth;
    var h = window.innerHeight || self.innerHeight || (de && de.clientHeight) || document.body.clientHeight;
    
    var div = document.getElementById("background");
    div.style.width = (w+38) + "px";
    div.style.height = (h-91-40) + "px";
    
    var count = Math.ceil(w/38) * Math.ceil(h/38);
    
    var newDiv;
    for(i = 0; i<count; i++) {
        newDiv = document.createElement("div");
        newDiv.setAttribute("class", "pic_" + getRandom(0, 11));
        div.appendChild(newDiv, div);
    }
}

function getRandom( min, max ) {
    if( min > max ) return( -1 );
    if( min == max ) return( min );
    return(min + parseInt(Math.random() * (max-min+1)));
}