something like this but shows the image
http://www.javafile.com/mouseover/linkdesc/linkdesc.php
just take alook around here I amsure you will find what you are looking for
it is javascript you wannt
<html>
<head>
<title>Image Pop Up On Option Change </title>
<script type="text/javascript">
function showpic(o) {
o = parseInt(o);
switch (o)
{
case 1:
imgName='del.gif';break;
case 2:
imgName='doc2.gif';break;
case 3:
imgName='docempty.gif';break;
case 4:
imgName='docurl.gif';break;
case 5:
imgName='new.gif';break;
}
var DivImage = document.getElementById('DivImg');
var DisplayImg = document.getElementById('IdImg');
DisplayImg.src=imgName;
DivImage.style.visibility="visible";
}
</script>
</head>
<body>
<form name="form2" method="post" action="race.php?action=race">
<table border="1"><td>Who do you wanna race?</td></table>
<select size=5 name="bike" onchange="showpic(this.options[this.selectedIndex].value)">
<option value="1" >Derbi Senda 50</option>
<option value="2">Honda NS 50 R</option>
<option value="3" >Suzuki ZR 50</option>
<option value="4">Yamaha DT 50 MX</option>
<option value="5">Aprilia RS 50</option>
</select>-<a href="java script:launchClasses()">?</a>
<input type=submit value=Register>
<div id="DivImg" style="position:relative;top:-50px;left:150px;z-index:20;width:25px;height:25px;visibility:hidden; text-align:center;"><img id="IdImg"></div>
</td>
</tr>
</form>
</body>
</html>
or this tutorial out of the link nielsieposted
http://www.javascriptkit.com/script/script2/htmltooltip.shtml