<html>
<head>
<script type="text/javascript">
function degistir(x){
var foto = document.getElementById("foto");
foto.src = x + '.jpg';
}
</script>
</head>
<body>
<div id="fotolar"><img id="foto" src="1.jpg" /></div>
<a href="#" onmouseover="degistir(2)" title="foto2">FOTO 2</a> -
<a href="#" onmouseover="degistir(3)" title="foto3">FOTO 3</a>
</body>
</html>