var floatImg = document.getElementById("oImg"); 
    var delay = 10; //控制每次执行间隔的时间，做越大移动得越慢；
    var speed = 1; //控制每次执行移动的距离，值越大移动得越快；
    var flagX = 0;
    var flagY = 0;
    
    function toPixel(str1) {
    //该函数用于去掉数值后面的px，并将之转化为数字。
        var oldLen = str1.length;
        var newLen = oldLen - 2;
        
        str2 = str1.slice(0, newLen);
        str3 = parseInt(str2);
        
        return str3;
    }
    
    function flowImg() {
        var bWidth = document.body.clientWidth;
        var bHeight = document.body.clientHeight;
        var bLeft = document.body.scrollLeft;
        var bTop = document.body.scrollTop;
        
        var iWidth = floatImg.offsetWidth;
        var iHeight = floatImg.offsetHeight;
        var iLeft = toPixel(floatImg.style.left);
        var iTop = toPixel(floatImg.style.top);
        
        //下面一段控制横向移动
        if(iLeft < (bWidth - iWidth) && flagX == 0) {
            floatImg.style.left = (iLeft + speed) + "px";
        }
        else if(iLeft >= (bWidth - iWidth) && flagX ==0) {
            flagX = 1;
        }
        else if(iLeft > 0 && flagX == 1) {
            floatImg.style.left = (iLeft - speed) + "px";
        }
        else if(0 >= iLeft && flagX == 1) {
            flagX = 0;
        }
        
        //下面一段控制纵向移动
        if(iTop < (bHeight - iHeight) && flagY == 0) {
            floatImg.style.top = (iTop + speed) + "px";
        }
        else if(iTop >= (bHeight - iHeight) && flagY ==0) {
            flagY = 1;
        }
        else if(iTop > 0 && flagY == 1) {
            floatImg.style.top = (iTop - speed) + "px";
        }
        else if(0 >= iTop && flagY == 1) {
            flagY = 0;
        }
    }
    
    var imgInterval = setInterval("flowImg()", delay);
    floatImg.onmouseover = function() {clearInterval(imgInterval);}
    floatImg.onmouseout = function() {imgInterval = setInterval("flowImg()", delay);}