DIV、IMG元素：

<div id="img" style="position: absolute;">
<img style="cursor: pointer;" src="pic.png" alt="img" width="200" height="100" border="0" />
</div>

JS代码：

var xPos = document.body.clientWidth-20;
var yPos = document.body.clientHeight/2;
var step = 1;
var delay = 9; 
var height = 0;
var Hoffset = 0;
var Woffset = 0;
var yon = 0;
var xon = 0;
var pause = true;
var interval;
img.style.top = yPos;
function changePos() {
	width = document.body.clientWidth;
	height = document.body.clientHeight;
	Hoffset = img.offsetHeight;
	Woffset = img.offsetWidth;
	img.style.left = xPos + document.body.scrollLeft;
	img.style.top = yPos + document.body.scrollTop;
	if (yon) {
		yPos = yPos + step;
	}else {
		yPos = yPos - step;
	}
	if (yPos < 0) { yon = 1; yPos = 0; } if (yPos >= (height - Hoffset)) {
		yon = 0;
		yPos = (height - Hoffset);
	}
	if (xon) {
		xPos = xPos + step;
	}else {
		xPos = xPos - step;
	}
	if (xPos < 0) { xon = 1; xPos = 0; } if (xPos >= (width - Woffset)) {
		xon = 0;
		xPos = (width - Woffset);
	}
}
function start() {
	img.visibility = "visible";
	interval = setInterval('changePos()', delay);
}
function pause_resume() {
	if(pause) {
		clearInterval(interval);
		pause = false;
	}else {
		interval = setInterval('changePos()',delay);
	pause = true;
	}
}
start();