<script ....>
    imgOff = new Image;
    imgOff.src = "path/to/image1";
 
    imgOn = new Image;
    imgOn.src = "/path/to/image2"; 
 </script>
 
 <a href="..." 
     onMouseOut="img.src=imgOff.src" 
     onMouseOver="img.src=imgOn.src">
   <img name="img" id="img" src="/path/to/image1"/>
 </a>