This script works in IE4 & Netscape 3.01 Gold. I'm not sure if it will work in any older browsers than Netscape 3. IE3 and below won't do anything on the mouseover.
This version is hard coded for the swap.
<a href="http://whatever/"
onMouseOver="rollover1.src='images/button.gif'"
onMouseOut="rollover1.src='images/loading.gif'">
<img name=rollover1 src="images/loading.gif" border="0">
</a>
The dot to the
left of this text is the image which is being preloaded and displayed
as small as possible (1 pixel x 1 pixel) to try to hide it. If the page
background is black then it should hide completely.