You actually need 2 events for this one, the onmouseover and onmouseout. The onmouseover will let you display text when the mouse pointer is on the image, while the onmouseout event will hide the text when your mouse pointer is out of the image. To illustrate further, let me show you some code.
<html>
<head>
<script type="text/javascript">
function mouseOver()
{
document.getElementById("lbl").innerHTML ="mouse over";
}
function mouseOut()
{
document.getElementById("lbl").innerHTML ="mouse out";
}
</script>
</head>
<body>
<img border="1" alt="My Image" src="image.gif" id="img1" onmouseover="mouseOver()" önmouseout="mouseOut()"/>
</img></body>
</html>
As you can see, I have pointed the two events of the image tag to the 2 JavaScript methods that will execute when the event has been fired.