Try putting it inside a layer. I'd use this code to acheive it

Code:
<div style="position:absolute; left:20px; top:50px; width:100px; height:100px;" onmouseover="document.getElementById('image1').style.visibility='visible'">
  <img id="image1" name="image1" src="image1.jpg" style="visibility:hidden;">
</div>