var floatImg = document.getElementById("img1"); 
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);}