Everytime you refesh your site, new picture will appear...
Save your pictures as:
image0.png
image1.png
image2.png
image3.png
image4.png
Code:
<html>

				<script type="text/Javascript">
					var randomNumber=Math.floor(Math.random()*5);
					var imagePath = "/;
					var theAd= imagePath+"image"+randomNumber+".png";
					document.write("<img src=\""+theAd+"\" />");
				</script>
<br /><br />
</div>	
</html>