I found one that works. http://javascript.internet.com/gener...ad-images.html
Code:
<!-- TWO STEPS TO INSTALL PRELOAD IMAGES:
1. Paste the first into the HEAD of your HTML document
2. Add the last code into the BODY of your HTML document -->
<!-- STEP ONE: Copy this code into the HEAD of your HTML document -->
<HEAD>
<SCRIPT LANGUAGE="JavaScript">
<!-- This script and many more are available free online at -->
<!-- The JavaScript Source!! http://javascript.internet.com -->
<!-- Begin
image1 = new Image();
image1.src = "http://www.hoistandcrane.com.au/testbed.gif";
// End -->
</script>
</HEAD>
<!-- STEP TWO: Insert this code into the BODY of your HTML document -->
<BODY>
<a href="http://www.hoistandcrane.com.au/mobile.htm" onmouseover="image1.src='http://www.hoistandcrane.com.au/testbed.gif';"
onmouseout="image1.src='http://www.hoistandcrane.com.au/test2.gif';">
<img name="image1" src="http://www.hoistandcrane.com.au/test2.gif" border=0></a>
</BODY>