function setBackgroundImageFromArray (arr)
{
    var i = Math.random() * arr.length;
    i = Math.floor(i);

    var style = document.body.style;

    style ['backgroundRepeat'] = 'repeat';
    style ['backgroundAttachment'] = 'scroll';
    style ['backgroundPosition'] = 'top left';
    style ['backgroundImage'] = 'url(' + arr [i] + ')';
}

function setBackgroundImageFromImages (images)
{
    var arr = [];

    for (var i = 0; i < images.length; i++)
    {
        arr.push (images [i].src);
    }

    setBackgroundImageFromArray (arr);
}

function setBackgroundImageFromTag (tagId)
{
    var imagesHolder = document.getElementById(tagId);

    if (!imagesHolder)
    {
        return;
    }

    var images = imagesHolder.getElementsByTagName('img');
    setBackgroundImageFromImages(images);
}