Código HTML:
<html>
<head>
<script>
function cambiar(url){
document.getElementById('imagen').src = url;
}
</script>
</head>
<img src="http://tbn0.google.com/images?q=tbn:7s4I2nAYqMA_zM:http://general.sc35.info/artlists/firefox_eats_ie.jpg" onmouseover="cambiar('http://general.sc35.info/artlists/firefox_eats_ie.jpg')">
<img src="http://tbn0.google.com/images?q=tbn:NxKg0-0Yvi_F9M:http://iconpacks.mozdev.org/images/firefox-experiment.jpg" onmouseover="cambiar('http://iconpacks.mozdev.org/images/firefox-experiment.jpg')">
<img src="http://tbn0.google.com/images?q=tbn:Hq2_Q7TdCJQcpM:http://www.solophotoshop.com/imgarticulos/397/firefox-solophotoshop.jpg" onmouseover="cambiar('http://www.solophotoshop.com/imgarticulos/397/firefox-solophotoshop.jpg')">
<BR>
<img src="http://general.sc35.info/artlists/firefox_eats_ie.jpg" id="imagen">
</body>
</html> Podés ver el resultado del código acá:
http://hgp147.110mb.com/codigo.htm