Yo no lo veo tan dificil:
Código:
<html>
<head>
<title>Untitled</title>
<script type="text/javascript">
var x=0; var y=0;
if(!document.all) {
document.captureEvents(Event.MOUSEMOVE);
}
document.onmousemove = show;
function crearAlt(texto) {
document.getElementById("ALTdHTML").innerHTML = "<img src='image001.gif' width='800' height='600'>";
}
function show(evento) {
document.getElementById("ALTdHTML").style.visibility = 'visible';
//document.getElementById("ALTdHTML").style.display = 'block';
if(document.all) {
x = event.clientX;
y = event.clientY + document.body.scrollTop;
document.getElementById("ALTdHTML").style.left = x + 10;
document.getElementById("ALTdHTML").style.top = y + 14;
} else {
x = evento.clientX + document.body.scrollLeft;
y = evento.clientY + document.body.scrollTop;
document.getElementById("ALTdHTML").style.left = x + 10;
document.getElementById("ALTdHTML").style.top = y + 14;
}
}
function hide() {
document.getElementById("ALTdHTML").style.visibility = 'hidden';
//document.getElementById("ALTdHTML").style.display = 'none';
document.getElementById("ALTdHTML").innerHTML = "";
}
</script>
</head>
<body >
<div id="ALTdHTML" style="position:absolute;width:291px;height:138px; top:137px;left:303px;visibility:hidden"></div>
<img src="image001.gif" width="235" height="139" onMouseOver="crearAlt()" onMouseOut="hide()">
</body>
</html>